To move over slowly while sitting or lying down, without using any limbs.
Locomotion is achieved by building upper-body speed up in a direction, then jerking the lower body all at once, thus exceeding the maximum static friction and sliding a short distance.
Also, to move an object one is sitting on without getting off of it by the same means. If you recall being in the supermarket as a kid, this is how you moved the cart those few feet over even though you didn't have anything to push off of.
Skoonch also applies to dragging oneself or something one is on top of, so long as the dragging is done purely by surface friction and not by grabbing any stable objects.