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From BR Bullpen
In a rundown situation, the baserunner will often try to extend the situation as long as possible, to allow teammates to advance further, as he knows he has little chance to escape safely unless there is an error. For its part, the defense will try to tag the runner quickly, preferably by chasing him back to the last base he occupied, thus minimizing the potential damage if the runner somehow manages to escape. The defense must also be careful not to commit obstruction, and a fielder who throws the ball must quickly get out of the way, lest he impedes the baserunner if he turns around. As a result, a number of fielders will be involved if a run down takes a while to complete, and the putout will read like a telephone number (for example 1-3-4-2-6-3, representing a runner caught between first base and second on a pick-off attempt, with the first baseman, both middle infielders and the catcher also involved in snuffing out the runner).
Both runners and fielders practice executing run downs during spring training.