From the 2016 PGA Championship at Baltusrol G.C. How to keep your left arm straight in your golf swing? In order to have width in your golf swing, you really would be well served to have good posture, and a good pivot. If a golfer has correct posture, then a good pivot is easier. A correct pivot promotes a full shoulder turn, which in turn provides width of arc, which most gofers see as a straight left (or lead) arm.