It sounds like what is being described is merely a slingshot motion; perfectly legal.

The "Chinese changup" that I am familiar with has the pitcher complete the windmill revolution after releasing the pitch. That is illegal.

The typical use that I see is an additional pitch to a pitcher with a full windmill motion. After the batters get used to seeing the backswing, full arms extended above the head, and a full windmill, the pitcher then uses this pitching motion, which releases the ball on the first forward motion.

It CAN be legal, if there is no continuing revolution of the arms to simulate the windmill after releasing the ball.

In 2003 FED fastpitch rules (7.2.2 EFFECT), the ball could hit the plate for a strike. In a silent and unpublicized editorial change, the 2004 NFHS rulebook deleted the words "in slow pitch". But, as noted previously, not in ASA, nor in NCAA.