After contact has been made with the baseball the hitter will follow through.  The batter will continue the swing path but slowly decelerate the bat speed.  The hitter does not want to stop their swing suddenly after making contact with the ball because the ball will not have any power behind it.  It would be like punching a person and stopping right when they make contact with the person.  It would hurt the person, but if they followed through they would have more power with the punch.  It is the same concept with swinging a baseball bat.  This is an example of elaboration.  The hitter will remember the example of a punch and will be more likely to remember to follow through on their swing.

Hitters often wonder if they should have two hands on the bat or just one when they follow through.  It does not matter as long as they keep both hands on the bat while contact is being made with the ball.  If the hitter takes one hand off the bat while still making contact, it will weaken the swing and the hitter will not have as much power.