This exercise works the muscles of the lower body and torso. It is especially good for runners and jumpers.
Assume a stance with one leg extended forward with the knee over the midpoint of the foot and the other leg back with the knee bent and a little behind shoulders. 
Jump as high and straight up as possible. Block with the arms to gain additional lift. At the apex of the jump reverse the position of the legs, that is, front to back and back to front. Switching the legs occurs in midair, and you must do it quickly before landing.
Upon landing, retain the spread-legged position, bending the knees to absorb the shock. It is important to keep the shoulders back and in line with the hips to maintain proper stability.