This is another exercise that I learned from P90X, and it's a brilliant one to help you improve the shape of your buttocks.
The fact that it's a lateral (side to side) movement means that the outer portions of your buttocks (your hips) are going to get the workout. For a more rounded look, this is one of those exercises you want to do!
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Lateral One-Legged Hops
Place something on the floor to mark the center line--a piece of tape, a sweater, a water bottle, etc. Stand to the right of the center line, with most of your weight resting on your right leg.
Leap to the left, over the center line, and land with your left leg only. Bring your right knee up to your chest, and don't let it touch the ground as you land.
Leap back to the right, over the center line, and land with your left knee bent and your right leg on the floor.
Repeat as desired!