Let's compare Rodriguez to Barry Bonds and [Albert] Pujols and see if we can figure out what's going on.
Rodriguez does something that the other two don't (or potentially in the case of Bonds "didn't") and the result is that he has a larger margin for error. This error margin is what holds him back from being even better.
When Bonds bats, his front foot hardly lifts off the ground. It moves just slightly forward while Bonds keeps his head and eyes perfectly still and maintains a low center of gravity. Pujols only lifts the heel of his front foot, while staying balanced and low. One result is maximized visual acuity. Another is the ability to get the front foot properly planted when it's time to attack the ball with the synergistic forces of the legs, hips, shoulders, arms and hands. Very seldom can a pitcher catch either Bonds or Pujols off balance enough to disrupt their swing.
Rodriguez is different. His stance begins balanced, low and stable. But as the pitcher releases the ball, Rodriguez starts an obtrusive attack with what I'm sure he thinks is a precise timing mechanism to generate a power surge. It isn't and it doesn't. What happens is that Rodriguez lifts his front foot high off the ground while he waits in suspended animation to detect the speed, direction and nuances of the pitch before he abruptly lunges forward and down to plant the foot so as to begin the swing. If the plant is too early, he's out in front of the pitch and loses much of his power. If he is late with the plant, the fast ball is by him.
There wasn't anybody on last night in the 12th inning when Rodriguez hit the go-ahead homer. No matter, it was a much-needed shot in the arm for both the team and Rodriguez, especially on a night when Mariano Rivera blew his first save of the year and Hank Steinbrenner all but conceeded the season.