A "double under" is a common jump rope maneuver in which a person rotates the jump rope underneath his feet twice, prior to landing on a jump. Double unders require significant coordination, fitness, timing and efficiency in moving the rope, according to Daily Burn.
Double unders are common in crossfit training programs. One key to successful double unders, especially with multiple repetitions, is to flick the rope with the wrist and not the arm, according to Men's Fitness. It often takes time to master the double under; failure not only frustrates the person performing the move, but it can lead to painful leg whips with the rope.