You proposed the top hand had no purpose at the point of contact because the drive already happened at "go".
I do think that we might agree when "go" starts but I think "go" lasts longer than you think.
Your clips certainly show that the decision to "go" happens when the pitch is beyond half way to the plate.
The hitters movement has already started as the pitcher still has the ball, but "go" as you call it happens after the hitters body has fluently reached the "launch" position.
The momentum of Posada's swing gradually builds and his "launch position" happens just as he stops his swing.
The majority of "GO" is driven by his back side (top hand, back leg, etc.), and it takes place from his "launch position" until his top hand comes off the bat. "Hitting Zone"