Thanks Jamesh.

A better example of the ball and rope with rotation.

Place the ball behind the back shoulder, have the lead wrist hinged back towards the forearm and pull a little down on the end of the rope and start rotating the lead arm.

Notice the speed at which it comes down and the wide swing arch of the rope.

When you read or watch someone who teaches both arms are straight at impact, 99% of the time they have used some form of rolling rotation.

Some people will curl the last 3 fingers of the lead hand on the way down, that is also a form of rolling rotation.

I am with the Old Timers on this one, since being so old: The ball and rope can give you a good feel and understanding for the swing you want to produce.