This is one of those gadgets that it's design is so simple its verging on genius. Chin- ups are great way of strengthening your upper body, basically because you use your arms/shoulders to lift your entire body off the ground.
Normally to do chin-ups at home you'd get a chin up bar which you place it in the door frame and screw it in to both sides so doesn't slip down when you apply weight to it. This is okay if you never want to shut the door.
The Powerbar is different because it just hooks on to the front and back of the door frame so you can just remove it at any time. When you're done it can even fold flat.