The thing that helps me with the firm left arm idea is that I don't have to manipulate the club to get it back to square at impact. I don't play enough and have enough time to practice enough to allow anything in my swing that requires an additional manipulation.

I'm sure I'm missing something, but I have to have the simplest swing possible in order to have something somewhat repeatable.

Having said that, I'm sure my "firm" left arm bends a bit, just as the shaft of the club bends a bit, but when I have the conscious feeling of firmness, or at least remind myself of that, I have my most consistent impact and best distance. As Paul said, it's a matter of physics.

Bill