This is a speed-strength, lower-body plyometric.
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 or more.
3. Immediately jump upward into the air as high as you can; when your feet hit the floor again, jump once more as high as you can, i.e., two jumps in sequence, hence the name the double rebound.
4. 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.
5. Return to starting position atop the box for beginning of next repetition.