Acid Rain is one of Lee's best moves. It's an i10 punisher that does 44 damage. The catch is that it's not one, but two just frames. On top of that, because it's a punisher, you need to account for the block stun.
Unlike b2 loops, the difference between Acid Rain and the alternatives (in particular 1,2,4 and 4,3) is significant. Not only do the easy options do much less damage, they're all minus on hit, whereas Acid Rain gives good oki. So learning Acid Rain is a higher priority than learning b2 loops.
However, like b2 loops, this optimization is still not the highest priority. You should only practise it once you feel confident you want to play Lee.
The first 3 can be input after the 1 jab's active frame. This delays it, which delays the input windows of the just frames as well.
As Imyourfather shows, there are two common methods: Adding a filler input ("wa-da-da-da-da"), and doing only the required inputs ("boo-doop boop boop").
The first method uses a simple rhythm with a tempo of 8.5 frames, so notes are on frames 0–8.5–17–25.5–34. For a lot of people this is easiest, since all you have to do is drill that tempo in and bang out the notes.
For the second method, the rhythm is not fixed so there are many variations. These rhythms opt to be less stable to avoid using a filler input. They may also happen to change the tempo, but a 10 frame tempo is no easier than a 8.5 frame one.
Punishers depend on the input buffer, which allows you to easily connect e.g., a 10 frame punisher after a -10 move. Without it, you'd need to time the punisher perfectly—too early and you get nothing, too late and they can block.
For the common Acid Rain methods, however, there's a problem. If you start the rhythm in the input buffer, the just frame windows will be off tempo.
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To use Acid Rain as a punisher with these methods, which is its main purpose, not only do you need to time the 3's perfectly, but you need to time the 1 perfectly after the block stun as well. This is a big problem, because different moves have different block stuns, and there's no way you can drill perfect timing for all of them. (That's why the game has an input buffer.)
One potential solution is to visually confirm the input—that is, press the second 3 when you see the first 3 connecting, then use a 10 frame tempo for the third 3. For this to work, however, you need a fixed input delay. If you only ever play offline and with the same setup, this might be a viable solution. But even then it's probably not advisable, since visual confirms are a lot harder than rhythms to train muscle memory for.
The rhythm 0–14–28–38 works both with and without an input buffer.
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Why does this work? In the input windows table, frame 0 is not always the frame when you press 1. If the 1 is buffered, frame 0 is pushed forward by however many frames it's buffered.
By delaying 1,3 and thus the just frame windows, we have room to account for frame 0 being pushed forward. For every frame of buffer, we lose a frame of delay, and our just frame windows stay in the same place. This works for up to 4 frames of buffer, since that's the most we can delay 1,3.
Fortunately, this isn't a difficult rhythm either.
Acid Rain itself deals 28 damage, but it also knocks them down right in front of you for a followup.
Guaranteed followups are:
- Wheel of Fate (u/b+3), dealing 16 damage with +20a (+3a) frame advantage.
- Blazing Kick (d,D/B+4), dealing 19 damage with +10 (-7a) frame advantage. The D/B+4 can't be buffered and the input window for it to be guaranteed is 6 frames. Many players don't bother with this since a late input gets you launched and it has worse oki anyway.
Extra options are:
- Pulse Blast (f,F+3), dealing 20 damage if the opponent stands up.
- Feather Landing (f+3+4), dealing 22 damage if the opponent stays down. Depending on the opponent's size, side rolls can cause this to whiff.
- You can get 47 damage with d,D/B+4 followup, but see #Followup.
- Or at least it should be, but Imyourfather's "boo-doop boop boop" is consistently off timing and doesn't match his button presses. The move still comes out because his voice isn't the one making the inputs. It's interesting that his muscle memory totally ignores the off-timed cue he's vocalizing.