Findings from Batspeed.com research show that rotational swing mechanics increase bat speed more than linear swing mechanics. The role of the lead arm in the swing is one of the main reasons for the differences in bat speed generation between the two swings. Linear mechanics has the batter's lead arm extending to contact. With rotational mechanics, the bend in the lead arm remains constant during rotation.
One of the keys to rotational hitting is the batter's lead arm mechanics. In order for the batter to attain maximum bat acceleration, the bend in his lead arm must remain at a fixed angle during rotation. Keeping the elbow at a fixed angle generates greater bat speed because it produces a more productive Circular-Hand-Path (CHP) and "Hook" effect.
Below are a couple of Batspeed.com Youtube videos that illustrates the importance of keeping the angle of the lead arm at a fixed angle during the swing.
In order to attain maximum acceleration of the bat approaching contact, the bottom-hand must apply a rearward force on the handle as the top-hand applies the forward force. To better illustrate this, lets use the analogy of loosening a nut with a 4-prong tire wrench. In order to loosen the nut, you would push up with your right-hand while pulling downward with your left-hand. Pushing up or pulling down with both hands would apply no torque to turn the nut. The forces must be from opposing directions to torque the nut loose.
Now lets use the 4-prong tire wrench analogy to your swing. Starting your swing with a bent lead-elbow and then straitening out the arm results in both hands being driven forward. Both hands driving in the same direction would apply little torque to accelerate the bat-head to contact. However, if you maintain the bend in your elbow (not extending the arm), the rotation of your lead-shoulder back toward the catcher will provide a rearward pull on the lead-hand you need to apply maximum torque