I am going to start posting more explanations of golf phrases you have heard and make sure to clarify them. Some may be repeats of comments I have made in longer posts, but I too often see golfers implementing cliches poorly.
I have heard for 25+ years you are supposed to have a straight left arm (for a right handed golfer). While I agree that a massively bent arm is horrible, a locked left arm is just as bad.
I have seen high handicappers, under the “expert” tutelage of a mid handicappers, being taught to have a straight left arm and it ends up being a stiff, hyper extended left elbow. That is really bad. Not only does it create tension, it usually creates a stiff left shoulder that gets buried into the neck and a stagnant shoulder turn…which is death.
I often change the wording of golf instruction lingo that just seems to be a difference of semantics, but it is not. The way something is worded can have a huge affect on the way it is utilized.
IMO, I think the phrase should be, “you don’t want a bent left arm.” You may want the left arm to be straight, but you want it to be soft and relaxed. A stiff, locked left arm is nothing but trouble.