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 impossibility 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.