Take position hanging from the pull-up bar with a medium or wide grip, whatever feels more comfortable. Let your body hang down freely. This is the starting position.
Commence the movement by contracting your abdominals and breathing out as you lift your legs from their straightened position up toward the ceiling. You may either bend the legs at the knee or keep the legs relatively straight as you pull them up to a point where they pass parallel to the floor. The higher you take them, the better the contraction.
At the top of the movement, squeeze the abs hard before slowly lowering the legs back to the starting position, inhaling as you do.