As good as your mechanics may be at various points in the golf swing, the chance of hitting a good shot is severely diminished when things go wrong at impact. The proper orientation of the left arm at impact, for example, depends not only on its relationship to the club, but also on its extension and placement in relation to the rest of the body.
Extension
Both arms should be extended fully at impact. This extension, however, should not be limited to the moment of contact with the ball. A good golf swing will use the full length of the arms with the hands extended away from the shoulders to create a wide swing arc, as described by Karen Palacios-Jansen on the website Golf Tips. Arms that are not fully extended will compromise the quality of contact between clubface and ball.
Sequence
Ben Hogan writes that the proper sequence in the downswing begins with the hips, followed by the shoulders which lead the arms and hands through to impact. A left arm that races ahead of the body in the downswing will end up in an improper position when the club face makes contact with the ball, usually causing shots to be pulled or hooked to the left.
Matching the Shaft
Michael Hebron, writing on Golf.com, points out that the left arm should form a straight line with the shaft of the club at the point of impact. This position represents the club head catching up with the rest of the swing just as it reaches the ball. Also, the left arm at this point in the swing will rest up against the upper chest to keep the club shaft at the proper angle into the follow through.
Release
The process of the club head catching up with the hands and arms at contact represents the release of the hands that must be delayed until well into the downswing, according to Ben Hogan. The release of the hands is accompanied by a slight counter-clockwise rotation of the lower left arm leading up to impact. It's generally easiest, however, to think of squaring the club face at impact to determine how much to roll the arms in the downswing.
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