The concept of multiple unders is fairly straight forward; the rope goes under your feet multiple times in a single jump. As simple as it sounds, it would actually require severe timing coordination of your hands and feet, and consume lots of practice. Unless you are skipping quadriple unders or higher, I can say that timing is more important than how fast you turn the rope or how high you jump.
Double Under

You should practice double under until you could do it with your shoulders relaxed.
Double Crossover

- start with a double under.
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When the rope is right above your head, cross your hands.
- The rope passes under your feet at this point.
- After forming a cross, turn wrists once in the opposite direction from the original rotation, so that the rope goes above your head.
- Your hands should be open all the way by the time the rope passes under your feet.
- Repeat steps 2 to 4.
Double Continuous Cross

- Start with a double under.
-
When the rope is right above your head, cross your hands.
- The rope passes under your feet at this point.
-
Turn your wrists in the opposite direction from the original rotation and keep going with it.
- Make sure that you turn your wrists twice in one jump.
Triple Under

The final goal for doing triple under should be to skip it multiple times and be able to relax your shoulders while doing so. As you can see in the image, I still have a pair of tightened up shoulders when it comes to doing triples.
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