OK now I am going to demonstrate how to do a jack knife. Its a little bit more of an advanced exercise than a crunch or a side crunch. There's two ways I'm going to demonstrate it: One for a level one, and one for a more difficult level two. We're gonna start with level one: Your legs are going to be bent, your arms are gonna be straight, and you're gonna try to extend your legs, and then at the same exact time, simultaneously, you're gonna reach forward with your arms staying straight. Notice I still exhale on the contraction as I bring the knees and then the arms up together, as I fold up. So you're working your upper and your lower abdominals with this exercise; that's why it's a little more advanced. And then level two, will be keeping your legs straight, which requires a little bit more resistance than the lower abdominals, little more advanced, and you're gonna keep your legs straight and your lower back flat on the floor so you're not arching that back to come up, and reach with your arms straight. you can even go a little higher if you want and reach more toward your ankles. All these are just a little bit more variations to make the exercise a little bit more challenging. And that is the proper way to do a jack knife.