You need to maintain whole body position, movement and balance when you are spinning.
The more tired your legs get, the more your body will depend on other body parts, particularly your torso. You'll get a great tummy workout, because your abs will need to support your upper body as you spin.
In a spinning class, just at the point where you want to fall off the bike and die, some spinning instructor with the face of a supermodel, the body of an Olympic sportswoman, and the hair of a Playboy bunny, and who hasn't even broken out into a sweat, will shout ...