Sit on an exercise ball and roll out until your upper back rests on it; hold one dumbbell with both hands, arms extended above your chest, hips in a bridge position.
Lower the dumbbell in an arc back over your head until your arms are roughly parallel to the floor.
Pull the dumbbell back up in an arc to the starting position, squeezing your chest and lats.
Repeat for the desired reps, keeping your hips elevated throughout.