We had some UCLA players run one of our practices and they emphasized entering your circle of focus (or bubble). What does that mean? Between pitches the defensive players are relaxed and stinging straight up. As the pitcher goes into her windup, they take a step forward into their ready position with glove down, glove foot slightly forward, and 100% focused in the batter. So to answer your question both feet are on the ground and they have stopped moving forward by the time the ball reaches home plate. If they were actually moving forward, by defintion one foot would be off the ground and it would take a split second faster to change directions to move left or right to field a ball. When both feet are planted on the ground, you can immediately move to the ball in any direction.
I use hula hoops in the dirt to teach the circle of concentration (focus) concept. Look at others sports and see what other athletes are doing. I liken playing 3rd base to a goalie in hockey. Rarely is anyone motionless.