Throwing to an unoccupied base!

[VaCoach]

The rule says a pitcher "may throw to an unoccupied base in an attempt to put out an advancing runner." Here is my question and if there is a rule covering this situation, please include it in your response.
Runners are in first and second, two outs, 3 balls and 2 strikes count on the batter. On the pitch the runners are running to third and second, can the right handed pitcher begin his pitching motion from the stretch, after raising his left foot and looking home, then step straight towards third base and throw to third in an attempt to put out the runner on second who has started to run as he lifts his front foot? If this is a legal move, why have I never seen it, especially at the high school level?