Whenever you jump, you can press up/down/W while in the air to dash downwards, and bounce on the floor to do a high jump. If you stand on the ground and press Up and W at the same time, the game will register that us a jump and downwards-dash on the same frame, and it will trigger a high jump immediately.

This can make it much easier to do high jumps, without having to time your jumps and dashes.

Note that on the autosplitter version, 'Z' key can also be used as an alternate jump button.