I rarely use the term “shorten” when it comes to the backswing. Often times this will lead a player into not completing their swing. Most of the great players of all time stressed the importance of completing their swing when under pressure. An incomplete swing usually results in poor rhythm which is the “glue” to a swinging motion. No matter who is swinging, if out of rhythm it’s a guess as to where the ball will go.
Most people who need a “shorter” swing actually have a breakdown in their structure which causes their swing to become too narrow. Although the swing looks “long” the club isn’t going to behave properly.
Usually the culprit is a lack of “extensor action” in the trail arm. The trail arm is one of the biggest keys to a sound backswing. The trail arm is directly responsible for maintaining the width of arc. Simply stated, try to keep your hands as far away from your head as possible. This is best achieved by applying pressure on your lead thumb by the palm of your trail hand. The pressure should be maintained throughout the motion. If someone has a width breakdown this correct action will probably feel as if their trail arm doesn’t fold going back. It actually will fold the right amount but the feel will be much different.
A great drill to feel extensor action is to put on a jacket or sweater without putting the lead arm in the sleeve. The sleeve will just hang limp or inert. Take the sleeve with your trail hand and stretch it until taught. This is extensor action. Notice how the trail arm is keeping the lead arm extended. Now make some slow backswing motions keeping the stretch in that sleeve. Feel this in your swing and you will be on your way!! Now at the top instead of your arms breaking down under the pressure of the swing they will hold rock solid and the momentum of the Clubhead will actually bend and load the shaft. It’s important to swing the club back as far as possible with solid extensor action. The swing will now “appear” shorter but it will be A LOT wider and potentially more powerful!