The "Circle-Change" is a variation of the changeup that has become very popular in recent years and is great to throw with the Blitzball. To throw a proper circle-change, you should make a small circle with your thumb and index finger (just like making an "OK" sign with your hand) and then grip the Blitzball across its seam with your remaining three fingers (middle, ring, and pinky-fingers). For better deception, you will want to grip the circle-change with the ball resting deeper into your palm.
Throw the circle change with the exact same delivery and arm motion as the fastball and you should have a very nice off-speed pitch to throw off a hitters timing. Some people like to turn the ball over a little upon release in order to get a nice little screwball "fade" (Pedro Martinez is a master at this). In order to do this, just let the ball roll off of your three fingers slightly in the direction of your pinky.