This bug allows you to be able to walk as fast as you would normally but while being crouched down. When you are on slabs and have a block above your head (making you crouch) and you have a trapdoor. if you activate the trapdoor while inside it cant push you down and make you fast?
Other players see you as crouching but you see yourself as standing up.
In the video it shows you from the players perspective and seeing someone else doing it. you have to stand under the part where you are automatically crouched to be able to activate this.