A stability ball, also called an exercise ball, allows you to do a number of exercises that efficiently strengthen and tone your abdominal muscles, and it's also a good way to stretch out your torso and improve flexibility.
The stability ball is always placed against the floor or wall for exercises that require additional strength for stability. It can also be an assistance to certain exercises for beginners.
Gravity is the main resistance factor when using this equipment. The ball itself is very lightweight so it provides little to no use when hold it.