Mike Malaska talks about the change of direction in a golf swing. He believes there is no "top of the swing," just a start and a finish.
There is a point and time in the backswing at the club transition point where inertia takes over, and you start on the downswing. This should be one fluid motion, so think of the "top of the swing" as "the club transition point."
If you change directions correctly, that's what drops your arms and gets the club coming down perfectly into the ball. You will come over the top if you change directions incorrectly or too late.
A great way to practice change of direction is to use an Eye Coach tennis machine, where you can practice your forehand with a tennis racket. As Mike swings and hits the ball, the racket goes back, and he moves forward.
That same timing and motion are identical to a golf swing. As Mike swings the golf club back, he steps forward. This change of direction is a continuous athletic motion. It is not wrong to practice the transition point, but when your mind thinks to swing to the top and start down, you won't be able to get to your left side soon enough.
This same concept applies to multiple sports, such as baseball, hockey, soccer, and golf.