Extensor Action

What is it?
Extensor action is a preferred way of achieving a straight left arm in the golfswing.

Why would you want to have a straight left arm in the first place?
For starters it provides some consistency because the distance between left shoulder socket and hand stays the same this way.

Why can't you just use the muscles of your left arm to straighten it?
You could do this, but doing it this way builds up a lot of tension in the arm (and potentially the shoulder and back). This tension hinders the ability to freely move, turn or hinge your forearm and wrist.

What does it do exactly:
Extensor action pulls the left thumb away from the left shoulder socket.

How do you apply extensor action?
Grip a club with your left hand. Your left thumb should be riding on top of the shaft, somewhere between 12 or 1 o'clock. Now add the right hand. The lifeline of the right palm should now be covering the left thumb.
Apply pressure (push) to the #1 pressure point (the area where the back of the right thumb starts) with the right palm/heel. You can do this by actively straightening the right arm at the elbow joint throughout the swing.