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" first appeared on CrossFit.com March 9, 2001. The Double Under takes a bit of coordination and determination to master. Those with less coordination will find it a little more difficult. The Double Under must be executed with more intensity in order to complete a high number of repetitions without mistake.