Lei stances

Snake

Focus: Using short range mixups to springboard into other, more damaging stances.
Movement: Forward focused.
Properties: Forward movement grants slight high evasion.
Animal step left: Dragon
Animal step right: Panther (especially good since it's a crouching stance that can deal with the lows that Snake cannot).

Key transitions to snake

1,1f: Lei's main 10f punisher. A whole flowchart of stances stems from here.
SS 2+3 or b_f2+3: Goes into Snake without whiffing an attack. Both have niche usage to side step otherwise tricky moves.
df2f: Used to set up oki.
d4d: Because of the many versions of the low (d4, d4d, d4,4) you can get mind games on all but the most experienced and sharp anti-Lei players. Also a useful way to enter Snake from a ground hitting low.
db4,u/d: +8 on hit, evasive on whiff while still launching the opponent on a clean hit (albeit for less damage than the DRU transition). Can also be done from Back Turned where the damage potential is much higher.

Key Moves from Snake

Snake 2 and follow ups: Snake 2,2,2f is a mid, mid, low string that go into full crouch or a Panther mixup, while stopping at the 1st or 2nd hit puts you in Dragon stance. Hard to hitconfirm, but you should be in the habit of mixing up the number of hits regardless.
Snake 4,1f: A low, high natural combo into an extremely effective Tiger mix up. The 1st stays in Snake, while the 2nd hit can whiff at max range.
Snake 121,2: High, mid, mid, high string. The final hit isn't a natural combo and can be ducked, but it's a safe wall splatter you can mix with Panther transitioning the 3rd hit.

Sitting Snake

Evasive gimmick (from db1 or Tiger 3f) that leads into an interruptable mid splat or low launcher. Awful stance apart from the confusion factor.

Dragon

Focus: Poking and Pressure
Movement: Stronger forward movement
Properties: movement grants slight high evasion
Animal step left: Tiger
Animal step right: Snake

Key transitions to Dragon

fn1,2 u_d : Hit confirm from the 1st punch will give you a strong dragon situation.
b2f: Very good on hit. The follow up b2 will make some players freeze.

Key Moves In Dragon

Dragon 4,1 and follow ups: 10f high kick with a very strong ch confirmable string. It also has a usable tiger transition (Dragon 4,1f). This one of the key strengths to dragon's pressure.
Dragon 2: A mid knockdown. +4 on block, high evasive, ch launcher. A strong tool when player start respecting your dragon 4,1 ch power. Another usable tiger transition with dragon 2f, but you toss away you're frames on block and your combo on ch.
Dragon 3: The low from dragon, +4s on hit. Leaves Lei in full crouch and only -13 on block. Don't ever use the transition to crane on a standing opponent.

Panther

Focus: oki and 50/50
Movement: Very strong forward movement, very poor backward movement
Properties: High crush until you press a button, holding forward gives a low parry
Animal step left: Snake
Animal step right: Crane

Key transitions to Panther

All are great chances to apply a mix up on hit

fn1,2,1 u_d
Snake 1,2,1f
Snake 2,2,2f

Key Moves In Panther

Panther 1: A great 12f tracking low poke. +3s on hit and only - 12 on block and you're left in full crouch on block or hit.
Panther 1~2: This the same low but with a high nc follow up. Knocks down, splats at certain ranges. Death on block.
Panther 3: A low crushing low. Hits grounded and the reason for panther's power in oki. You're left in panther unless you hold back and enter phoenix.-12 on block, -1 on hit, +14g on ch.
Panther 2: The scary mid that makes people not want to knock the insane low. 15f launcher, big damage starter and only -10 on block. Even hits grounded at some angles.
Panther 1+4: His Rage drive from panther. turns his low, high move into a low, high launcher and and only -14. They will be forced to block the high if they block the low. This grants panther strong come back power.

Tiger

Focus: 50/50 and whiff punishment
Movement: Backward movement is stronger
Properties: Holding forward will parry high/mid punches and kicks. (+7)
Animal step left: Snake
Animal step right: Dragon

Key transitions to Tiger

Snake 4,1f: Main way to enter Tiger with a real mix up attempt without use of oki.
d/f+1+2f: Forces full crouch. They can interrupt your mix up with a ws4 but spacing and usage of parry can counter this. Also very strong for setting up oki in some situations.
fn4,1f: Decent situation on hit. Also the main way you'll use to use tiger RD for combos.

Key Moves In Tiger

All moves from Tiger are covered.

Tiger 1: A i16 safe mid that hits very low. CH launcher. If your opponent never side steps or spaces Tiger mix ups, this is a great safe option to go for. +2s on hit or block.
Tiger 2,2: With practice this move can become a HC launcher. Tracking on this quite strong. Tiger 2 by itself is -14 and the follow up is high. the 1st hit also has some high evasion.
Tiger 3: A low crushing mid. -12 on block. Knocks down/wall splats on nh. This will be an answer to people down jabbing your other tools and to go for wall splats.
Tiger 3f: This is a follow transition to sitting snake. You can try to mess up their block punishment attempt/whiff more safely this way.
Tiger 4: The only low from the stance. Arguably the best low in the game for its type. Death on block. Its range is outstanding, fully high crushes and hits grounded. NH launcher and will launch them correctly even if it hits their back. Completely unsaleable at i16.
Tiger 1+2: Safe mid wall bounce. Its on the slower end at i25 but can be used to whiff punish moves like get up kicks by the wall.
Tiger 1+4: Tiger RD. Turns your tiger 1 into a nh super launcher. The reason why lei's rage comebacks after a wall combo are so common. Hits semi grounded. +15g on block.

Crane

Focus: Wall Oki
Movement: Bad
Properties: auto low crush launcher
Animal step left: Panther
Animal step right: Snake

Key transitions to Crane

u/f+3: low crush on frame 6. Mid, ch launcher that leaves you in crane. +11 on hit, safe on block. Setting up frames traps with all crane moves except crane 1 in hit.
ub4~f: Used to set up crane oki after Lei's bnb wall combo.
fn1,2,1,2,4u_d: Decent chance to apply a 50/50 in the open if the mid kick connects.

Key Moves In Crane

Crane 1: 22F mid, knockdown, wall splat. On block, axis shift, leaves you BT and +5.
Crane 4: 21F low, hits grounded. Nh Knock down, free uf2 on CH. -19 on block.
Crane 3: 14F mid, has a delayable string. If Crane hits a true side axis opponent, the full string is free becoming a launcher.
Crane 1+2: 13F high Launcher, strong range. Whiff punisher on the rare occasion you bait an attack with Crane's poor movement.
Crane 2: 16F high, Homing. Ballerina Spin on hit, Wall Splats. Screw move on airborne. Ch launcher.

Phoenix

Phoenix is mostly used for combos. It does have one thing over the other stances for use outside of combos. It has a mid power crush normal hit launcher. It also has Lei's unblockable, which can be used in serval legitimate ways. Though by no means comparable to the strongest unblockables in the game. It also has a way to regain some health. The defensive property in this stance is shared with Drunken. Tapping forward will parry high punches and put you in Drunken for a free launcher. You can't move backward in this stance. When you try to move forward you end up in Drunken. Moves go into Back Turn, Snake, Slide and Drunken.

Drunken

Drunken is a stance that succeeds the most by the amount of options it has. Because it has access to all four of Lei's grounded stances as well being able to cancel out of the stance so fast that the opponent won't see Lei leave the stance, your opponent will have to worry about more moves than any other stance. This is the other stance that allows Lei to recover his health besides Phoenix. Along with Dragon, this stance has a unique throw in this stance. Because this stance has access to the throw, which gives a free follow up by the wall and a safe wall splatting mid, its super usable by the wall. The unorthodox ways Lei can go into the stance has utility. The speed in which he cover ground can be used to run away from the opponent if there is already some distance between you. The stance has inverted direction controls when in Back Turn Drunken. F3+4 goes into the stance raw and has a punch parry built in. Normally, Lei can't access his other moves that aren't a d, d/f or a d/b notation without canceling the stance. But Lei can do f+3+4 without need for a cancel. Taping forward will have a small window where it parry high punches and give Lei a free juggle.

Can be BT.

Backturn

This stance is used for one Lei's bnb combos. Outside of combos, it provides Lei with great mix up tools. High crushing, low, i12 ch launcher and a nh unseeable low launcher (i20) along with a few different types of mids. Because unique Lei rules for Back Turn, he moves toward the opponent when holding forward rather than turning around. This unique back turn is also what allows for what is Lei's most iconic move, the Haha step. In addition, he can enter Back Turn crouch back repeatedly in order to create large amounts of space. This is an non-optional state after some of Lei's punishers. The stance can side step into the background into front turn and you will get access to your side step moves. You can enter any of the grounded stances from Back Turn.

Grounded Stances

Lei has a unique relationship with downed stances. When opponent is downed a lot of moves whiff, Lei takes advantage of that and he is able to manually go down, and attack from there. But his grounded stances are not the same as downed stances, he can alternate between them using 2. But because of that players use specific names to each stance.

Knockdown

Knockdown (or KND) is entered using d+3+4, and is a stance alternate to FUFT. It's called like that because it's the most common state you will enter when being knocked down. Game calls d+3+4 and d+2+3 stances Play Dead, but only d+2+3 between players is called Play Dead for a specific reason.

Face Down

Face Down (or FCD) is entered using d+1+2, and is a stance alternate to FDFT. It's called like that as d+1+2 and d+3+4 can switch into each other by using 1. So Face Down stance is a Knockdown stance, but face down. In game d+1+2 and d+1+4 are called Sidewind, players don't use that name for no apparent reason

Play Dead

Play Dead (or PLD) is entered using d+2+3, and is a stance alternate to FUFA. It's called like that because historically (before Tekken 6) d+3+4 which game called Play Dead entered FUFA (after Tekken 6 it enters FUFT), thus the name stuck

Slide

Slide (or SLD) is entered using d+1+4, and is a stance alternate to FDFA. It's called like that because it gives access to Lei's manual Slide. The true reasoning behind that or any other naming is unknown