There are many types of advice golfers receive which are detrimental and could be called golf myths. Here are three examples: keep your left arm perfectly straight, keep your head still and shift your weight.
I am going to talk about problems these three things cause and the truth rather than the myth.
Many, but not all tour players do keep their left arms perfectly straight. Annika Sorenstam is one of those players. The average golfer is not in the physical condition that tour players maintain and lack the ability to do so. If you lack flexibility or have a bulky torso and arms, you will not be able to keep the left arm perfectly straight.
Attempting to do so will create tension and stiffness, sometimes injury to the shoulder area due to extending beyond what your shoulder can tolerate. A more appropriate thought is keeping your arm extended without locking it. Bob Toski refers to this as a soft extension.
The truth about the head is we do not want excessive head sway, but attempting to keep the head from moving at all prevents weight transfer and a proper turn. You need to make an athletic pivot which causes the head to move.
The head rotates and moves behind the ball during the back swing. The average tour player moves 2-4 inches and has several degrees of head rotation.
Thinking of shifting your weight often causes the hips to sway laterally. Joanne Carner called this the kiss of death. The truth is you need to make an athletic pivot which causes the weight to move.
An athletic pivot is what a basketball player will do in pivoting around the plant leg, as will a tennis player, when they need to change directions. Planting the leg and trying to shift onto it usually causes the hips to move laterally. Basketball, tennis and golf all need the pivot to transfer weight.
Golfers who combine these three thoughts are setting themselves up for a very upright, outside in, slice prone golf swing. Keep the head perfectly still, left arm locked and stiff, and shift the weight is a prescription for frustration.
Learn to make the pivot which causes the head to rotate and move a little while keeping the left arm extended, and you will automatically transfer your weight which will allow you to create a flatter, wider, more powerful golf swing and better golf shots.