Controller: Flick the stick in a horizontal direction then quickly flick it in the other direction and do this repeatedly to do a dash dance.

Keyboard: Tap A or D then quickly tap the other and do this repeatedly to do a dash dance.

Dash Dancing is a very useful technique for evading attacks, counter attacking quickly, putting pressure on the opponent, and in conjunction with other movement techniques, can create a confusing illusion that makes it extremely difficult for your opponents to land their attacks.

If you want to know more about Dashing-Dancing you can watch these helpful videos on the topic: