Set-up: Set-up either on a hanging-leg raise apparatus or a pull up bar. Using the appartus, grab the handles and press your lower back into the support. Using the pull-up bar, either form a tight grip on the bar or attach lifting straps to the bar and hang from the elbows.
Lift: Lift your knees to your chest, then lower them under control.
Emphasis: Stability and control are key on this exercise, or the lower back is subjected to unnecessary stress. Do not use momentum.