There are three major parts to a modified fast pitch: the starting position, the backswing (up to the top of the windup), and the delivery (down to and through the release).
A. Starting Position - The pitcher must: (1) Start with at least one foot on the pitching rubber. (2) Have his hips and shoulders fully square to home plate. (3) Pause while holding the ball in front of his body. (ASA requires a 1-10 second pause before the pitch. We require a similarly brief, but distinct pause. The intent of the rule is to prevent the pitcher from gaining unfair momentum for the pitch or quick pitching a batter.) The pitcher may hold the ball in either the glove or the pitching hand.
B. Backswing - The pitcher must: (1) Start the backswing directly out of the paused position. (2) Move the pitching hand backwards to start the delivery. The pitcher may: (1) Take the ball behind his back or up to up to the 12 o'clock position during the backswing. (2) Lift his front foot off the ground. (3) Slide or drag his pivot foot, so long as it remains in constant contact with the pitching rubber. The pitcher may not: (1) Step backwards or sideways off the rubber. (2) Use a "rocker" motion to start the delivery. (3) Use a "windmill" motion. (4) Use a "slingshot" motion - bending the elbow AND rotating the body towards first or third base. (All pitchers bend their elbows and turn their bodies to some extent. In an illegal slingshot, these movements are very pronounced so as to "whip" the ball around.) (5) Let the pitching hand go further than six (6) inches from the thigh. (6) Let the pitching hand come into contact with any part of the body during the backswing.
C. Delivery - The pitcher must: (1) Deliver the ball in a full underhand motion. (Sidearm pitches are illegal.) (2) Release the pitch on the first forward movement past the hip. The release must have a complete smooth follow through - with no abrupt stop of the arm near the hip. (3) Square the shoulders to home plate at the point of release. (Cross body, slingshot deliveries are illegal.) The pitcher May: (1) Take one forward step. The foot must come down within the 24" width of the pitching rubber. (2) Throw any "legal" softball pitch, such as a curveball, knuckleball, drop ball, etc. (Needless to say, foreign substances are illegal.) (3) Point the palm downward upon release. The pitcher may not: (1) Have the ball significantly outside the wrist during the downswing and release. (2) Use a "crow-hop" delivery. (A crow-hop delivery is defined by the ASA as jumping off the pitching rubber and re-planting the back foot prior to releasing the ball. It is legal to leap off the rubber during delivery so long as the pitch is released while in the air and the pitcher does not re-plant the pivot foot.) (3) Snap the wrist upon release. (This is a common feature of sling pitching, which is illegal.) (4) Let the pitching hand come into contact with any part of the body during the delivery.