The first base coach stands down the first base line in foul territory and gives advice to players. He is usually an assistant coach or knowledgeable player at the younger levels. In the professional ranks, the first base coach is most often a former player with years of knowledge and experience who knows the right advice to give to a baserunner in every situation.
Know Situation
The first base coach must be aware of the entire context of the baseball game: the score, the count, the number of outs, runners on base, tendencies of the pitcher, ability of defenders and the speed of runners. This knowledge combined with where the ball is hit allows him to give the best advice possible to a runner approaching first base.
Ball in Play
When the ball is hit, the batter will take off toward first base. To allow the batter to focus on running full speed, the first base coach is in charge of making the decision of whether or not to send the batter to second base. By knowing where the ball is hit an analyzing the situation, you can use the team's designated arm signals to tell the runner to stop at first base, continue toward second or round first and take a big lead, watching and reacting to how the play develops.
Leading Off
When there is a runner on first base, it is the responsibility to instruct the runner about the tendencies he has noticed about the pitcher, as well as verify the current situation, including count and outs. The first base coach will often say things like "get more" during a leadoff, instructing the runner to extend his lead a little farther. You should also be very loud when there is a pickoff attempt coming from the pitcher or catcher, making sure the runner hears you say "back" when the throw is coming.
Communication
The first base coach is also important between innings, when you can have a conversation with your players about things you have noticed about the pitcher, catcher or any of the infielders. The coaches can also offer encouragement and constructive criticism to players who might have made an excellent play or a mental mistake. If you discuss a potential steal situation in the dugout between innings with the players, you can avoid having your signs or signals potentially stolen by the other team.



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