MLB batters like @nicke.yorke above actually begin their swing just as the ball is leaving the pitchers hand. They need to develop maximum bat speed prior to impact and still be able to check up if the ball is not in the strike zone.
Phil Clarke below (Vanderbilt/BlueJays) practices this regularly with the ProVelocity Bat. An audible double click indicates he has obtained his pre-set bat speed prior to checking up.
D1 commit @jrsoliz_boom does the same in the on deck circle and then hits bombs when the ball is over the plate. (He has a recorded 105 mph exit velocity!)