Maintaining Powerslide
Mechanism / Why It Happens
You need to shift your weight backward to make the wheels slide. But this also creates a tendency for your body to fall backward.
First, to push the wheels forward while overcoming friction with the ground, you need to pull your body weight in the opposite direction of travel. This shifts your body weight far toward the heel side of the board, so if you stay in that position, you will fall backward. As you can see, this position cannot be maintained for very long on its own.

What to Do
Use the inertia acting on your body while you are moving.
A powerslide requires a certain amount of speed in the first place. This is because while friction w