The hamstring roll-in on a yoga ball begins with you lying on your back and placing your heels on top of the ball. Lift your hips off the ground, keeping your body in a straight line from shoulders to heels. Then, engage your hamstrings to roll the ball towards you by bending your knees, drawing the ball as close to your buttocks as possible. Finally, extend your legs back out, rolling the ball away from you to return to the starting position, while maintaining control and stability throughout the movement.