The follow-through is the result of the prior body movements. After impact it's just a matter of releasing all tension and letting centrifugal force finish your swing. Centrifugal force will keep the clubface travel and clubface angle in the proper position. After the ball leaves the clubface there is nothing you can do that will affect the flight of the ball.

The reason for good extension in the follow through is to keep the clubface accelerating at impact! The reason that the slice is so common is that many players try to keep the clubface angle square to the target through and after impact. This actually opens the face, resulting in a left to right spin to the ball (Slice).

Keeping the clubface angle square to the target after impact breaks down the left wrist, restricting the left arm rotation. These faults set up a chain reaction creating other faults. The lack of arm rotation affects both clubface direction and angle, resulting in both loss of accuracy and distance. Finally, your swing is going at such a high speed that it's a physical impossibilty for you to even attempt to make any adjustments so commit yourself to doing the right things properly in your setup and takeaway because after that things are out of your control.

Here's hoping you stay on the happy side of par!