Double jumping is when you jump once on the ground and once in the air making you jump higher than usual.

Why does it work: we don't know yet

How does it work: If you tap the jump button you will see that you will ofcourse jump, but if you tap it twice very fast you will get the double jump. Hypertapping is the best way to tap that fast.

In a bit more detail: Tapping the jump button will render after 4 frames or so and to do a double jump you have to tap the jump button again after the jump has rendered. After the jump has rendered there is a 5 frame (we don't know for sure) window in which you can tap again to get the second jump, if you tap in the first frame possible you will receive a jump higher than all of the other frames you can jump on. This is extremely hard to do with only two taps so I advice spamming.