Sit on a bench gripping the front edge, extend the legs forward with heels off the floor, and lean your upper body back to get close to a straight line. Bend and lift the knees toward your chest as you bring your chest forward, then lean back and extend the legs again.
 
Purpose
The bench jack knife is an easier variation of the jack knife that can be used as a progression to the jack knife or as a good finisher at high or max reps after higher intensity trunk work.
 
Programming
The bench jack knife should be performed for sets of 20+ reps and will generally be more of a stamina exercise with higher reps.
 
Variations
The bench jack knife can be done with a pause at the top and/or bottom, and with a single knee raise. Weight can be added by laying a plate on the shins.