a question regarding v-cancelling

so i know what exactly has to be done in order for a v-cancel to work, but im still in the dark on what exactly is happening when the v cancel occurs. so you press L or R 3 frames before the incoming attack, and that reduces the knockback by 5%; great. but why though? my guess was because your character isnt invincible for the first few frames of your airdodge animation, but the airdodge itself is still registered and therefore counts as a pseudo-tech if you will. im pretty sure im wrong but id just like a more thorough explanation, really :p thanks