There are many variations of the Jack Knife Crunch, each targeting different muscles of the core.
The Straight Leg Jack Knife Crunch targets the abdominals, primarily. Start by lying on your back and extending your legs out straight and arms overhead. Then simultaneously raise the arms to meet the legs above the hips as you contract the abs.
The Bent Leg Jack Knife Crunch puts less pressure on your spine but is still effective for the abs. Begin with the knees bent and raised. Then extend the legs as you simultaneously reach for the feet with the hands.
The next variation is the Single Leg Alternating Jack Knife Crunch which continues to target the abs but also engages the obliques. Start by lying on your back with both legs and arm extended. Bring the arms up to meet the opposite leg and alternate sides.
The final variation is a Side Lying Jack Knife Crunch which targets primarily the obliques. Begin lying on your side with your bottom arm wrapped around your waist and your top elbow bent with your fingers at your ears. Then simultaneously raise your legs and upper body as you contract your obliques and exhale. The range of motion is small, but the results are big.