At impact, the great players have their left (lead) arm internally rotated with their left (lead) elbow pointed at the target. It’s commonly believed that the forearms should “roll over” through impact such that the left (lead) elbow points more at the lead hip, but it’s not what the great players do.
Jordan Spieth’s left arm is slightly flexed at impact. It is a common misunderstanding to think that the left arm has to be straight at impact. In fact, we see many great players with bent left arms at impact.
Golf instruction tends to focus on superficial aspects of the swing (e.g., position at the top, keeping head still), almost all of which are irrelevant. What’s unique about the greatest players of all time (and what’s important to focus on when you’re trying to improve) is the core engine that they all have in common.
People might say, “Drive for show and putt for dough,” but it’s not true. Power is very important. The dominant players throughout the history of the game tend to be longer (in some cases, much longer) than their peers.
All great players have exceptional rotation. Some move a little bit laterally (like Hogan and Tiger), but no great players have significant lateral movement. Think of trying to close a door but the hinge is moving away – makes closing the door much tougher.
The evidence doesn’t bear this out. In fact, there’s more evidence that a longer backswing is better and more accurate. There are plenty of great players with long backswings, but here’s Ben Hogan, considered by many to be the best ball striker of all time, taking the club way past parallel on this driver backswing.