The elbow to knee is an isolation exercise that targets your obliques - the muscles on the sides of your abs. it also works on the rectus abdominus (the six pack) and hip flexors as a secondary muscles.
Working on your obliques will strengthen your core, and improve your posture and movement.
This exercise does not require any equipment and can be done at your home.
Do not pull your neck with your hands this will cause neck pains, make sure to touch it only lightly. Also, avoid touching your chest with your chin.
Make sure to maintain a natural spine. As most crunches, this exercise puts some pressure on your back, be careful if you suffer from lower back pain.