You learn that move in one of the later levels... I haven't checked out the demo yet, so I'm not sure how many levels are included. In the full version, you learn a new move with almost every new level (until you get to about level 10, after which there are no new moves).
The double spin kick, when you learn it, is performed by crouching and then pressing kick right after you let go of crouch (and begin to stand up).
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