Having coached for 20 years and having just witnessed, as a referee, many soccer games at our town's Columbus Day tournament, I again reaffirm my long-standing unscientific survey of games by noting that the teams that do the best are the one with the quietest coaches. Why is this?
Soccer is a thinking person's game, and it is hard for a player to think if an authority figure is yelling at you as the ball comes your way. Kids who are trained to think learn how to make the right decisions in the split-second action of a game. Kids who are trained to listen to their coaches learn to wait to be told what to do.
Here's what I was taught by a great coach, Dean Conway, in coaching school and try to pass along to my fellow coaches. You coach during practices or quietly on the sidelines to the players who are waiting to be substituted in. You do not yell instructions to players on the field -- especially ones near the ball -- because (1) by the time you yell something, the play has developed and your instruction is too late; (2) chances are that your instruction was wrong in the first instance, anyway; and (3) if the player is listening to you, she is not able to think for herself or does not hear a teammate calling for the pass or otherwise saying something important.
Coaches who are reading this and don't believe me should hear what the kids say to each other and to me (as referee) on the field when their coaches persist in yelling instructions. Trust me, their comments about you are not pretty.
As a coach in a tournament, I love it when the opposing coach yells instructions. Two things happen. First, I see the other team's players get all tense, make mistakes, and lose their sense of teamwork. Second, my kids turn to me and say, "Can you believe that guy?" Then they (not I) win the game.