If I stand at the driving range and I look at the golfers practicing, I can see very clearly that most of the players tend to keep the left elbow as straight as they can throughout the entire swing.
The perfect sequence on the downswing has been discussed for the longest time, and till now it is still a common discussion between Biomechanics .
Some of them think that the sequence has to be from the ground up. . Others that is not important if the upper body start first on the downswing compared to the lower body .
However the greatest news is that thanks to the 3D technology like MySwing, we are able to see through the biomechanics graphs if the left elbow and the other segments are adding or limiting the speed during the swing (kinematic sequence graph).
There are 4 body components that create an impressive angular speed if they are correctly applied in sequence: pelvis (1), torax (2), left arm (3) and club (4) .
What we firmly know is that the most efficient swing is not the same for every golfer, efficiency is unique to your body. By adding adding another link in the sequence and using it in the correct timing we can create more speed . The result is going to be awesome!
Phil Cheetham ran a research on 57 Tour Players and found that 42% are following the normal 1-2-3-4 sequence, 35% are following the 1-3-2-4 sequence and the rest are. following uncommon combinations, the interesting part is that Regardless what sequence they follow, 86% of these players have their pelvis starting first at the beginning of the downswing, the longest drivers tent to bend the left elbow adding another link to the sequence specifically between phase 3 and 4 changing the sequence to pelvis (1), torax (2), left arm (3), left elbow (4) and club (5). The Pelvis has to transfer his own speed to the thorax, the thorax then has to pass the speed to the arm and on like that till the speed reaches the club .
If you are able to release the left elbow at the correct time, it will act as another link in the chain and add significant speed to the swing.
I have always been teaching my students to bend their left arm during the backswing also for another two reasons: first of all because if you don't, the left shoulder and the scapula become rigid creating less rotation around the spine with less flexibility, second of all because the rigidity can cause the release of the wrist in the early downswing (casting) and loss of energy .
The best combination is to reach the top of the backswing feeling the left elbow slightly bent, which will allow you to have a good rotation and to have a chance to add another link to your chain.
It’s all about timing rather than how much you bend the elbow.
For reference, the average Tour players have their left arm at the top of the backswing at about 144° (calculated on 180° when your elbow is straight), basically the elbow is bended at an average of 36°.
Any change you do with your body needs to go through the forces and torque of the hands in order to have an influence on the club head speed.
I suggest all the readers to work on these 3 aspects to achieve significant improvement in the forces transmitted to the segments, increasing the club speed .