From BR Bullpen
A force out (aka force play) is made when a baserunner is forced to leave his base when the batter becomes a baserunner himself, and a fielder successfully tags the next base before the runner can reach it. No run can score on a play in which the final out of the inning is recorded on a force out, even if the runner crossed the plate before the final out was recorded.
From the official rules:
- 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. 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.
- Any runner is out when ... (e) He fails to reach the next base before a fielder tags him or the base, after he has been forced to advance by reason of the batter becoming a runner. However, if a following runner is put out on a force play, the force is removed and the runner must be tagged to be put out.