For successful double unders you need consistent control. You must control the rhythm of your jump, the speed of your rope, and most importantly you must control your body. Common double under technique mistakes that work against this include piking and donkey kicking.
Piking is a mistake because it involves unnecessarily contracting a lot of muscles (abdominals, hip flexors, neck flexors, etc) which will increase the rate at which you fatigue. On top of that, most people that pike during double unders will end up slowly jumping backwards as they do so. This causes your jump and rope path to be inconsistent.
Donkey kicking not only adds more unnecessary muscle activation, but as you can see from the picture above it brings your feet even higher off the ground. Combine your feet moving down from the kick at a high speed along with gravity pulling you down and you end up jamming your feet into the ground and not being able to “bounce” off your toes.
Now, I know everyone reading this is asking, “That’s great, but HOW can I fix these problems!”
For starters, relax. Start by just doing some single unders to make sure you do not pike or donkey kick. Next you can start practicing jumping higher, with or without a rope. If you feel your body piking or doing a donkey kick then stop, take a breath, and relax. Keep everything loose and stay on your toes as you bounce. Once you master controlling your body you’ll find you are using much less energy and double unders will become less taxing.