Fresno, CA - Thomas Myers in his book Anatomy Trains talks about the Front & Back Functional Lines (FFL/BFL) and Spiral Fascial Line (SFL), which connect the right shoulder to the left hip, and left shoulder to right hip.  Think about a big black 'X' printed on the chest and back. 

Thomas Myers goes on to say about the SFL: 

"The overall movement function of the SFL is to create and mediate spirals and rotations in the body, and, in eccentric and isometric contraction, to steady the trunk and leg to keep it from folding into rotational collapse."

This creates what Joey Myers from the Hitting Performance Lab calls a "Springy X pattern".  Joey Myers goes on to say,

"A hitter loads the body like a spring.  For example, a lefty like Adrian Gonzalez would load this "Springy X-Pattern" by taking his front shoulder down and in towards his back hip."

Not all Big Leaguers do this effectively, but

Adrian Gonzalez is having a very good year as a 33 year old steady producer. He's a big slugger, 6-2 and 225 according to FanGraphs. He has an .894 OPS, good for 7th in the league.

What Adrian Gonzalez does better than most big sluggers before the stride foot lands, is he uses his torso to generate power by:

1. Showing his numbers to the pitcher,
2. Hiding his hands from the pitcher, and
3. Having a slight downward shoulder angle...

Joey Myers refers to this as the Catapult Loading System.

This activates what Thomas Myers calls the FFL/BFL & SFL, which helps generate bat speed by tapping into the body's natural springy loading mechanisms.  It's also a power system the body is very good at repeating explosive rotational movements.  Hence, A-Gon's ability to weather the peaks and valleys of power hitting over the course of a 162-game season.