Weighted Russian Twist

Adding a dumbbell here is really about taking away your ability to cheat — the extra load makes it harder to fling the rotation with momentum, so you're forced to control the turn instead of throwing it. Sit with knees bent, lean back to about 45 degrees, chest up, and hold the dumbbell with both hands at your chest. Rotate your torso side to side, tapping the weight lightly near the floor beside each hip while your hips and feet stay still. If you find yourself rocking your whole upper body instead of twisting through your ribs, drop the weight down until the rotation comes back.