Only the upper part of Varitek’s pocket is loose. The lower part of the pocket (where a catcher is supposed to receive the baseball) is all one piece with the rest of the palm of the glove (see link). I know sometimes a catcher has to catch a ball further up in the area of the glove that is loose on Varitek’s mitt, but most of the time this should have no effect on how the catcher receives the ball. In fact, if you watch Varitek catch, you can see the upper part of the pocket flopping around when the ball hits his mitt. This wouldn’t happen if he caught the ball further up in the loose area of the pocket.
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