A tech trap is any technique that gets free damage if the opponent does a tech recovery. The most common way to do this is jumping over a tech roll so that the defender recovers back turned (and is therefore unable to guard).
Tech traps can be powerful since techs are supposed to be the most robust wake-up option. However, setting up a tech trap often sacrifices too much combo damage to be worth going for outside of a comeback situation.
Generic tech traps
Jump over at wall
Most characters can do a tech trap off a wall splat when the wall is off-axis by around 30 degrees or more. If the defender techs at this angle and the attacker jumps forwards with UF, they'll both end up back turned. Depending on the angle, this often only traps tech rolls in a single direction (e.g. traps left techs but not right).
The easiest way to get the timing right for the jump is doing a 2-hit wall combo where the last hit recovers fast. Most characters have a string for this, and jab into df+1 also usually works.
The simplest way to make use of this jump over is getting a guaranteed BT.d+1. After the BT.d+1 the attacker is +9 with the defender still back turned. From here, i14 while standing moves are guaranteed. i15 moves can hit if the defender holds B, but they can be blocked if the defender holds F.
Basic example of the jump over dickjab tech trap.
i15 WS followup hitting defender holding B.
i15 WS followup blocked by defender holding F.
The followups aren't necessarily guaranteed vs characters with special BT moves such as Xiaoyu (who has a BT parry).
Jumping crossup kick
It's possible to get a crossup with a jumping 4 at the wall. This works because jumping 4 has a large enough inner hitbox to hit the defender in the side-ish (but just behind enough that they're actually in BT).
(The author doesn't yet have enough understanding of this trap to clarify further.)