A FORCE PLAY is a play in which a runner legally loses his right to occupy a base by reason of the batter becoming a runner.
Rule 2.00 (Force Play) Comment: Confusion regarding this play is removed by remembering that frequently the “force” situation is removed during the play. Example: Man on first, one out, ball hit sharply to first baseman who touches the bag and batter-runner is out. The force is removed at that moment and runner advancing to second must be tagged. If there had been a runner on third or second, and either of these runners scored before the tag-out at second, the run counts. Had the first baseman thrown to second and the ball then had been returned to first, the play at second was a force out, making two outs, and the return throw to first ahead of the runner would have made three outs. In that case, no run would score.