This exercise develops core function, strength, and control in a manner similar to various tricks with Freeskiing. Very similar to a mute grab while hanging from a pull-up bar, the exercise requires contraction of the core for stability, combined with hip flexion and a vertical body position to bring the legs up to the upper body, which contrasts from many supine/ground core exercises.
To start: find a vertical bench at your gym. (You can also do this exercise on a pull-up bar.).