This is a speed-strength, lower-body plyometric. There is often confusion between a height jump and a depth jump.
1. Begin by standing atop an 18 inch or taller box (if you have no experience with this exercise, use a standard bench such as one for bench presses, etc.).
2. Jump off the box and flex your knees so that when you hit the floor/ground that they bend 3-4 inches.
3. Take care when you jump not to be stiff legged or to land flat footed; land on the balls of your feet, flexing your knees.
4. Return to starting position atop the box for beginning of next repetition.