The Jackknife pull-up is a fantastic exercise for a couple of reasons. First, it can be used as a bridge exercise to help you on the journey to your first full body-weight pull-up. Second, it is a kind of hybrid between the horizontal pull and the vertical pull movement patterns, which are two essential movements in the pulling family. The horizontal pull includes the row, where your feet are on the ground or a raised surface and your body is roughly parallel to the ground. The vertical pull includes the pull-up and the chin-up, among others
If you are training for your first pull-up
Start with rings or suspension trainers at about armpit height. Grasp the rings and position yourself so your legs are out in front of you and your body is an L shape. If you straighten your legs out and your body is roughly parallel to the ground you are doing a row. The more you bend at the waist, the closer you are coming to the true Jackknife pull-up. You can start with rows and as you get stronger, move closer and closer to the Jackknife position. When you get comfortable here, it is time to elevate your feet.
If using this for your main pulling exercise
Find an object to put your feet on that is between 2-3 feet high. The higher it is, the more difficult the exercise will be. Place the object relatively close to the rings or trainers, probably about 2 feet away. The closer the object is to the trainers, the more vertical your upper body will be when performing the movement. This more closely approximates the pull-up and will be the most difficult variation. Don’t worry about keeping your legs straight but make sure you are bending at the waist as much as you can. Try to keep that Jackknife position. You can use your legs to some degree to assist you with the movement.