[Mathew]

On a more serious note - the whole thing about the delivery line preparation at the top (view Yoda's video on this) is so your in a position to uncock and roll on the plane line...

Now basically this means for swingers is your left hand in a position at the top of the backstroke to be able to do a left hand karate chop towards and along that plane line.

Some pro's practice left arm only shots to monitor this...