“ A tech [...] is an action that can be performed when the player's character hits the ground, a wall, or a ceiling while tumbling. To tech, the user must press a shield button a certain period of time before hitting the surface. [...] A successful tech grants the performer intangibility during most of its animation, but leaves them completely vulnerable for a few frames at the end of the animation, after which the character becomes actionable again. — SSB Wiki ”
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Teching works the same for Fox as for everyone else, but because he can be punished so hard, missing a tech can be lethal. Teching is not conceptually difficult, but even high-level players often miss techs, especially from fast and unexpected attacks like shine or due to tech traps.