In baseball, a circle changeup is a pitch thrown with a grip that includes a circle formation, hence the name. The circle is formed by putting the index fingertip against the thumb tip, and holding the ball against the palm with the remaining three fingers. This pitch will tend to have little ball rotation. The circle changeup is a variation of the straight changeup.
A circle change can also be used to provide screwball-type movement. By placing the index and ring fingers slightly to the inside (that is, towards the thumb) of the ball and sharply pronating the forearm at release, a pitcher can make the ball move downward and armside. A left-handed pitcher's circle change will break down and away from a right-handed batter. Effective circle changeups can reduce the platoon split a pitcher will experience.
To follow proper form, release the ball keeping your wrist straight, then follow through fully. If this pitch is placed too high in the strike zone, it can be hit very hard. It is an effective pitch to throw early in the count to produce a groundball; it is not traditionally used to acquire a strikeout. By rotating your wrist (before you release) you can change the movement from resembling a fastball to resembling a curveball.
You must throw it like a fastball, so you do not tip off the batter.
It is incorrect to think of a changeup as nothing more than a "slow pitch." It is much more sophisticated than that. It is an illusion. The pitcher uses the same, fast arm speed that he uses with his fastball except the ball comes at the batter with a much reduced velocity. There is a seeming disconnect between what the batter sees from the pitcher's motion and the resultant velocity of the ball. The pitcher accomplishes this with the grip. The "circle" made by the index finger and the thumb is designed to weaken the pitcher's grip. This allows him to throw the ball very hard and achieve a slower velocity.
Much like how Mariano Rivera has made his key pitch the cut fastball, Trevor Hoffman will credit many, if not all, of his 510 saves to his superb changeup, although he throws his with a palmball grip. Despite Hoffman's fastball's drastic decline in velocity, he remains one of baseball's elite closers, due to his stellar changeup. Eric Gagné will credit his circle changeup while mixing in his fastball for his success although arm injuries have troubled him for the past two seasons. Nolan Ryan is another famous player known for this changeup. Two Minnesota aces Frank Viola and Johan Santana, both Cy Young award winners credit much of their success to this changeup. Also on the list is Pedro Martinez, who rode to three Cy Youngs and some of the most dominant pitching performances partially due to his outstanding circle change.