This is a three-part exercise with a ball between your knees.
To start: face the bar. If you don't have a bar you can use a piece of wood or two books that are equal in height.
Place the ball between your knees and place your toes on the bar while keeping the heels on the floor.
Lift your arm as high as you can, then reach down towards your toes.
As you reach down, push your hips back, keeping your back straight.
Next, step on the bar with your heels keeping the toes on the ground, and squeeze the ball in between your knees.
Reach down toward your toes, then stand up straight, again keeping a flat back throughout.
Finally, place your feet flat on the ground and repeat the movement reaching up and down whilst squeezing the ball between your knees.