(1) The pitcher starts with both feet on the rubber.
(2) The pitcher must hold the ball in both hands for a second before starting her windup.
(3) The pitcher can do a back swing before starting her pitching arm forward.
(4) The pitcher can't throw "side armed".
(5) If the pitcher pushes off aggressively, she has to drag the "push off foot" on the ground. This means that a pitcher can't leap toward the batter.
(6) A pitcher can't "crow hop"--a pitcher can't jump forward, reset her pivot foot, and then throw.
Generally, umpires call (5) or (6) only in extreme situations.
A crow hop is defined as the act of a pitcher who steps, hops or drags off the front foot of the pitcher's plate, replants the pivot foot, establishing a second impetus (or starting point), pushes off from the newly established starting point and completes delivery
Pushing off with the pivot foot from a place other than the pitcher's plate is illegal. This includes the "crow hop" as defined under Rule 1.
(Women's and all JO Play) Pushing off and dragging the pivot foot in contact with the ground is required. If a hole has been created, the pivot foot may drag no higher than the level plane of the ground.
If a pitcher pushes off the rubber and her weight comes down on her pivot foot again prior to stride foot landing (similar to what a great deal of pitchers do today), what criteria does the umpire use to determine whether the pitcher used a second push, this time from a new starting point (impetus)?
One thing umpires look for is the angle of the pivot foot as it retouches. Even if it touches prior to the stride foot coming down if it is pointed toe down it's not considered a replant. If it touches on the ball of the foot you might get called for a replant, if you land on your heel and roll to the ball of your foot, it's an easy call as a replant
Here is a link to some NFHS illegal pitches with videos