When a guard "pulls," it's a deliberate attempt to force his defensive counterpart to one side or the other, as opposed to blocking straight ahead. The intent is to open up a lane for the running back between either the guard and the center (to spring the runner straight up the middle), or between the guard and the offensive tackle. If the tackle pulls in the same direction as the guard, they're trying to open up an outside run, or an "end-around."
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