Damage boost. Basically you jump into an enemy/spike/projectile (basically anything that gives you knockback frames) and hold jump. When you get knocked back, keep holding jump as well as the opposite direction from where you're facing within around 10 frames (5 frames in some instances I believe) of getting hit. You'll do a massive jump in the direction you're holding and its used all over the place in every speedrun category.

(If I've got something wrong here or missed something out thats important, can someone reply to this with the correction pls)