The shortstop is the infield leader. He calls the pop-ups, makes sure coverage is set for double plays or for bunts, and if he's off his game everyone could be in trouble.
Second baseman may not have quite the arm, but in ever other aspect needs to be as sharp. This page shows how both should handle grounders straight at them and line drive leaps.
GROUNDERS

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Be Ready - sometimes the most important move (shown on the left) is before the pitch, before the hit. Stay on the balls of your feet, knees loose, glove down, and take a couple of steps in on each pitch.
The catch, transfer, and arm action must be kept quick and contained...
LINE DRIVE LEAPS

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Techniques for leaps really depend on direction of ball flight, and your ability to get some spring in your jump. Work on Plyometric exercizes.
DIVING CATCHES

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Time your dive to reach the hit while still high enough off the dirt to recover - or expect the ball to pop loose..