1) You must drive the legs and finish in a reverse C-looking follow through.
2) The swing is controlled by the left arm, and the right should be kept out of it altogether.  (This applies to right handed golfers.  I will periodically use the terms target arm and trail arm to accommodate the lefty’s). 

This article is about the illusion of the quiet trail arm.

One of my coaches went so far as to have me swing a heavy sledgehammer with my left arm only (I am a right-hand golfer).  I looked like a ½ Popeye; my left forearm was huge, a good deal larger than my right.

I hit thousands of balls with target arm only, and got so I could hit it pretty well that way.

But problems began to develop.

For one thing, I began to develop tendonitis in the wrist and shoulder, which I battle with to this day.  Swing problems also snuck in: my swing got shorter and shorter, quicker and quicker.  It also got steep, too up and down.  The natural swing I grew up with had become choppy.

Thank goodness the conventional wisdom began to change, and golfers began to use both arms again.  But to this day, golfers blame overuse of the trail arm for problems that are actually from incorrect use, not over-use.

The golf swing can be described as a side-arm throwing motion, a lot like skipping a flat stone on a lake.  Most right-hand golfers also throw with the right hand, (not all; the hockey influence makes Canada an exception), so using this throwing image can simplify and clarify your swing thoughts.

In the backswing, the trail arm folds, but not too much.  Think of a box at the top, or as the old Scottish pro suggested, a waiter holding a tray of food.  Better yet, if you were in the shower, turned sideways so that the curtain rod was behind you, and you reached back to hold the rod, your trail arm would be in the correct position, with the club (rod) perfectly on-plane.

A correct feeling would also be of the right arm pushing out on the left.  If you have trouble keeping your lead arm straight, it is because the trail arm isn’t pushing it.

In the downswing, the trail arm begins to straighten and drop underneath your shoulders, with both arms going straight just after impact.  The straightening of the right arm supplies a great deal of the punch in a good swing.

A FANTASTIC EXERCISE

Get a thin but strong rope, about 3 feet long.  Tie a loop in one end, about as big around as your arm at the shoulder.  Slip the loop over your target arm, holding the loop in place with said hand where the arm meets the shoulder.

Hold the other end of the rope with your trail hand, making the rope taut.

Swing back and make the box, keeping the rope taut.  That rope represents your target arm.  Notice how it is straight. The trail arm straightens the target arm.

Coming back down, keep the rope taut, gradually straightening the arm.  Just past impact it will be fully extended, still putting pressure on the rope.  Keep the rope taught to belt-high, at which point the arms can begin to fold.

Then, put a 7-iron and the rope both in the trail hand, and do the same thing, slowly. Once you get the idea, put a ball down and hit a few one-handed, with the rope as your new target arm.

Train the trail arm in this way, and you might discover a missing piece to your golf-swing puzzle.