In melee, in order to tech you must have digitally pressed L/R within 20f before hitting the ground (see: meleemechanics video). After every digital L/R input (including shielding) the game registers a 40f downtime. If you press L/R again within these 40f then you will not tech and the 40f downtime will be refreshed. This is designed to harshly punish mashing tech inputs.
A techtrap is a method of exploiting this downtime to ensure that a second tech is not possible.
Jigglypuff’s upthrow upair under platforms is her most recognizable techtrap. Should a fast faller attempt to tech the upthrow then the upair’s low KB will delay the ground collision just enough that he will still be in tech downtime and thus unable to tech when he hits the platform, then you rest him. In order to tech the upair the spacie must choose to not tech the upthrow. Naturally, this leaves Jigglypuff with the option to simply not upair and to rest the resulting missed tech on upthrow. In this way techtraps at a high level are a mixup.
In order to eliminate false techtraps an intelligent player will buffer the first tech early and input the second tech late. Most people do this intuitively. Executed well this minimizes the trap timing to 20f. For a late techtrap to be “real” the hit must connect as closely as possible to the ground collision without it occurring and opponent must then collide with the ground within 20f of the original timing (realistically I think that you can confidently estimate for 30f before your gamble on their imperfect execution gets testy).
LATE TECHTRAPS on ffers include (% before hit, approximate)
upsmash at 15%
dash attack ~30-60%
you can punish people for inputting techs early by timing your hit early. This is where techtraps get wonky and interesting. If you hit someone earlier (at about 15-20f before they hit the ground) then it will eat up their tech input (they won’t be hitting the ground within 20f) and it makes your techtraps more reliable over a greater % range by virtue of having a full 40f of downtime. This is made even more lenient because you can use hitlag frames as a window to cover later timings (this is exactly why upsmash works well in practice). Basically early techtraps are superior to late techtraps in that they have the same weakness (they lose to no tech input) but work over wider windows.
EARLY TECHTRAPS on ffers include (% before hit, approximate)
upsmash at 15-30%
dash attack ~30-70%
Note that some of these last longer than you could possibly follow up.
Note too that you can combo to rest instead of waiting for the tech trap to play out and then jab reset resting. This is all relatively situational but it’s cool that once more melee aligns itself into an elegant system.
This is an unlikely mixup compared to simply not teching but should an opponent attempt to counter your early techtraps by delaying his input (you can usually hear it from their controller) then you should just delay your hits.
Niche funny thing: Because shielding activates tech downtime for 40f Puff’s fthrow/bthrow on spacies will reliably techtrap under 40% if you grab as the shield comes up but they will go far enough from the throw that you’re not going to be able to get there to jab. There are DI dependent setups to get a platform techchase from fthrow on floaties with fthrow. This is more practical with other characters that have fast throws (ex: Marth’s dthrow).