Instructions:

1. Sit on a bench or chair with your feet flat on the ground and your back straight.
2. Hold a dumbbell in one hand with an underhand grip, resting your forearm on your thigh, palm facing up.
3. Allow the dumbbell to roll down to your fingertips, then curl it back up by flexing your fingers.
4. Repeat for the desired number of repetitions, then switch to the other hand.