When learning, a lot of players look like this the pitcher on the right: leaning forward and her back leg has not moved towards the target by the time the ball is being released. When a pitcher throws “all arm”, this is what is happening mechanically. 

What’s more powerful, sprinting forward or jumping sideways? Just as a sprinter leaves the block, pitchers are sprinters leaving the mound. The fastest way to run to the finish line is forward, not sideways. 

If a pitcher can put herself in a good position at 3 o’clock (top pictures), she sets herself up for the rest of the motion. If not, it can be very difficult to correct her mistakes mid-pitch.