Speed Rope.

Double-unders are an equipment question too.

Double-unders are about timing, and timing requires a rope that rotates the same way every single time. The ball bearings in the handles give smooth rotation without catching, the cable is nylon-coated steel wire that adjusts to your height, and the knurled aluminium handles stay in the hand even when it is sweaty. The whole rope weighs 160 grams. Small equipment. High pulse.