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dude it's raining
Xeylode is right, knockback that causes tumble (>= 80, for reference sheiks fair had tumble kb vs pal fox starting 38% before hit) or being in the tumble animation is required for you to be able to tech. Hitstun doesn't matter at all actually, you can still tech if you are out of hitstun, but usually when you tech you are still in hitstun. Also when falling backwards from edge during shielding, you're put into 26 frame
missfoot animation, during which you can't do anything except ff. Then after that you go into tumble if you are still in the air, and can tech or do anything else. You land normally during
missfoot and get knocked down during tumble.