I have seen many people on the forums and on the community Discord asking how to do this glitch, so here is an explanation.

Step 1: Get into E State. E State is achieved by pressing down and reset at the same time.

Step 2: Immediately after starting the level, jump up and dive onto the ground before the bounce pad. Your character should stand up with its sprite half stuck in the floor. If you turn on hitboxes, your hitbox will be standing horizontally on the ground.

Step 3: Walk towards the bounce pad so that it bounces you up. Then, move left in midair to the wall. Before you hit the wall, release left and hit right for a short period of time. You will know if you did this correctly if your character is in the UWJ animation.

Step 4: Spam up and smash right before you hit the spikes. This is frame perfect and may take a few tries to get right.