Jump switching, as opposed to passively changing men, means that you jump out violently on the man with the ball. You don’t wait for him—you go out and get him. A jump switch commonly occurs when a dribbler tries to use a screen. The defender guarding the screener jumps out and takes the dribbler with one very specific objective: He wants to prevent any dribbles in the direction of the screen. In other words, the defender jumps out on the dribbler and either stops him immediately or turns him back in the direction he came from. He does not play passively so that the dribbler can dribble past the screen and continue on. You do not want the ball to continue on, because this allows the screener to roll free to the basket for an easy pass.
From Dick’s book Stuff
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