To perform a one-leg hamstring roll-in on a yoga ball, start by lying on your back with your knees bent and your feet resting on top of the ball. Extend one leg straight out while keeping the other foot firmly planted on the ball. Engage your core and use your hips to roll the ball towards your buttocks, bending your knee and bringing the ball closer to you. Then, slowly extend your leg back out to the starting position, maintaining control throughout the movement. Repeat for the desired number of repetitions, then switch legs. This exercise targets the hamstrings, glutes, and core while also challenging balance and stability.