Timing the Pitcher
The base runner must practice her timing leaving the base for a lead off or steal. If she leaves that bag before the pitch leaves the pitchers hand, she will be called out for leaving the base too early. For this reason the player must practice leaving the base at the pitch. For all practical purposes the time between when a pitchers arm is at the top of her circle to when she is releasing is less than a second
For this reason, you will often see a girl use a rocker step to leave the base. This involves putting the push off foot at the front of the base with the stride foot at the back of the base. When the pitcher gets to the top of the circle, the runner begins her forward motion with the stride foot with the push off foot leaving the bag behind the stride foot. If the runner starts this motion at the top of the windmill, then typically the pitch is released by the time the push off foot leaves the bag. This timing must be practiced until it becomes routine.
The rocker step is not required. Some girls put the stride foot forward and time not making a move with the push off foot until the pitch is released. This is a matter of taste of the runner, but I prefer the rocker step and insist upon my girls using it. I like the timing of it and it seems to me to create a quicker momentum toward the next base.