Tag up

In baseball, to tag up is for a baserunner to retouch or remain on their starting base (the time-of-pitch base) until (after) the ball is first touched by a fielder. By rule, baserunners must tag up when a hit ball is caught by a fielder before it bounces, and in such situations, the baserunners are out if any fielder with possession of the ball touches their starting base before they do. After a legal tag up, runners are free to try to advance, even if the ball was caught in foul territory. On long fly ball outs, runners can often gain a base. When a runner scores by these means, it is known as a sacrifice fly. On short fly balls, runners rarely attempt to advance after tagging up, due to the high risk of being thrown out.