With a 10- to 25-pound weight in your hand, place the top of your left foot squarely on the stability ball, with your right foot a couple feet out in front, right knee slightly bent.
Fall at your waist as you roll the ball backward with your left leg until your back is parallel to the ground, letting the weight hang straight down. Make sure not to reach with the weight; you don't want your back rounding at any time during the exercise.
Squeeze your bottom as you roll the ball back to the starting position and raise your body upright, keeping your core engaged for stability.
Repeat this move 10 to 15 times before switching sides.