Hi,

A slight bending of the left arm at the top of the backswing is not a bad thing, actually a crescent type shape from the left shoulder to the wrist is a very good and powerful position.

Where bending becomes a problem and a big power leak is when the elbows collapse resulting in a narrow back and downswing.

Here is a link to a video lesson that explains:

http://www.ritson-sole.com/golf-tips...ng-your-power/