Skull crushers: You hold the bar in a pronated grip position over your head/chest, while lying on your back. Keeping the elbows in and pointed at the ceiling, you lower the bar to (I prefer slightly behind) the forehead. Then extend the arms, kinda like you are doing a pressdown. Same basic principle, except lying on your back with weights in your hand. This exercise is pretty hard to just describe, and I don't know of any sites with an illustration.