At contact, the front leg locks while the back leg and hip explode toward the ball. Front arm is fully extended and the back arm and hand remain behind the bat. Head is pointed toward contact. The bat continues along the path of the ball to correct the batter's error in timing. The head is released upward, allowing a complete follow-through.
Some players release the top hand.
Some players elect to keep the bat vertical for as long as possible through the swing. The bat appears lighter and the swing is quicker. Although flattening the bat makes it feel heavier, some hitters do so. This gets the bat on the plane of the ball earlier. The grip becomes firm at contact.