A dewsweeper writes in that says they have been told two different things and are confused. One that they should swing down the line nad one that they should swing the club around to the left (they are a right handed golfer) after impact which is correct.

Well first lets discuss what really happens in the golf swing. The swing is a circle and because of that the club must swing back up the arc to the left after impact.
The club only hits the target line for a split second and that is at impact.
what happens when a person swings down the line is that they become diconnected with their arms and their trunk. they tend to swing the club too much to off the plane to the right
they will lose power and be forced to use their hands to get the ball started back on the proper line.
this will reduce their power and their consistency.
tools you can use to practice swinging the club around the circle:
lay a small hula hoop on the groun and hit balls standing in the middle of it let your hands feel as if they travel around the hoop after impact with the ball.
this will help you visualize the correct arc rather than swinging down the line.