"To tag up is to retouch or remain on the runner's time-of-pitch base until (after) the ball either lands in fair territory or is first touched by a fielder. A runner must, by rule, tag up only when a batted ball is caught by a fielder (without touching the ground). After a legal tag up, even if the ball was caught in foul territory, runners are free to attempt to advance."
"A force is a situation when a baserunner is compelled (or forced) to vacate his time-of-pitch base—and thus try to advance to the next base—because the batter became a runner. A runner at first base is always forced to attempt to advance to second base when the batter becomes a runner. Runners at second or third base are forced only when all bases preceding their time-of-pitch base are occupied by other baserunners and the batter becomes a runner. A forced runner is out (called a force out) when a fielder with the ball touches the runner's force base before the runner reaches that base. A forced runner also may be tagged out in the usual fashion as well; such a tag is still considered a force play if the tag is made before the runner reaches his force base."
