I would be willing to bet (pretty high stakes, too) that Pena does not think about exaggerating his back knee push when he is swinging at a pitched baseball in a major league game. My opinion is that his knee scraping the ground is a result of learned motor behavior, and for him to do this, is quite natural FOR HIM.
Would exaggerating knee push/pull help bat speed? I would say, quite possible, but how do propose pulling it off? Would you actually make a conscious effort to direct your knee to do this while you are at bat? Or would you practice the movement in a batting cage, hoping that it would become an involuntary act while swinging?