The Jackknife is a great abdominal exercise which requires a good amount of strength to perform.
Muscles used: rectus abdominus, psoas, obliques
Start: you should be lying supine (on your back) on a mat. The arms should be extended behind you and on the floor and the legs should be fully extended and together.
Movement: the movement consists of two movements performed together. Part one is to curl your upper body up with your arms guiding the body through the movement. You should only need to curl up a couple of inches off of the ground. (a common mistake is to sit up too high, be careful of that)
Part two of the movement is to simultaneously lift the legs up in the air. Keep the knees as straight as possible. Attempt to touch your toes, ankles or knees.
After touching, go back to the starting position and repeat.
Ryan George Fitness 
If one is not flexible enough to touch their toes… is it better to bend the legs in order to get them up higher or keep them straight but not get the as far off the floor?