The legs should remain flexed at all times during the swing. On the backswing, the right leg shouldn't lock, it shouldn't even straighten. The same is true of the left leg on the forward part of the swing. There used to be a school of thought that claimed it was desirable to hit against a straight left leg. It is quite an incorrect principle.
If there has been any one thing that has bothered me most in my efforts to improve my swing and hit better shots, it has been getting away from a long-ingrained habit of straightening the left leg as I come into the ball. If your left leg straightens in the impact area, your left side pulls up. This stops the left side from turning as it should as you hit through the ball. It throws your swing off the proper line and it checks the speed of your swing. Even at the finish of your swing when the left leg is necessarily straight, it isn't locked.
The key for me in correcting this habit of straightening the left leg was thinking of "staying level." By this I mean the feeling I get that my legs and my knees remain level with each other, that there is no up-and-down motion by either until I have completed the swing. I concentrate on this and I just continue turning.
FRED HAWKINS, El Paso Country Club, Texas