Grab all the gems you see so you can enter the next level. You move your upper body with the arrow keys and when you grab on to another wall, let go of the arrow and your behind will quickly catch up. Obviously it is not this simple, there are several traps set up to make it more difficult, like live wire or exploding balls. You do need to avoid these. Don't try to hold on to the metal tiles, you can't and your body will go all the way back.
Use the arrow or the WASD keys to extend your body.