A double under is a popular exercise done on a jump rope in which the rope makes two passes per jump instead of just one. It is significantly more effective than a single rope pass in that it allows for higher work capacity.
The Double Under takes a bit of coordination and determination to master. Those with less coordination will find it a little more difficult.
The main key to Double Under success is learning how to control the rope. Rope control will help you coordinate your jumps to the whipping of your wrist.