When you're looking to target the abs, try and minimize the involvement of the hip flexors. The only way to do this is to make sure most of the movement is from the flexion and extension of your spine, not from the hips.
Coaching Tips
The bent-knee variation works best. The straight-leg version will change the movement in such a way that the rectus femorus (quad) and the hip flexor (psoas) will be stressed the most, and even more so when hamstring flexibility isn't there to allow a full range of motion.
Always start in the top position with your arms straight out in front of you next to your lower legs. Bend your knees and make sure your shins are horizontal. Keeping your knees and feet level will shift your center of mass, guaranteeing abdominal recruitment.
Start by opening up (extending) your whole body. As you reach your arms overhead, drop your torso back and down and straighten the legs. Once you reach the fully extended position, start the concentric part of the movement by rolling your spine up as you bring your arms over and in front. At the same time, pull your knees into your chest while keeping the shins level with the floor – don't pull your heels to your butt. Make sure you have complete balance and control in this position before starting your next rep.