The left arm is kept "straight" (not locked) all through the swing until just after impact when it begins to fold. as Hogan writes in his book:
"In order for the club to travel its maximum arc, one arm must be extended at all times." he goes on to say: "In general, then, the left arm should be straight, this doesn't mean that it should be locked at the wrist or elbow or be in any place as the arm of a robot." unnatural straining isn't t at all necessary, or desirable.
Now, that being said.....I find that for me to maintain a straight left arm, I have to consciously "stretch it out" away from the body during take away. I use Steve Stricker as a model. I am sure though that after it reaches a horizontal position hip high, that there may be a slight bend in my left arm, but it is still fairly straight. My hardest thing is to get that straight left arm up above my shoulder and still keep my right elbow tucked. Maybe I need to work on the shoulder turn, I don't know.