Several people have asked what exactly the Center Armrest Extension is and does.
You simply press the button in the front center and lift lightly until the rest locks into the high position. Then you press the button again and push the armrest back and down and it locks into the low position. It lets you use the rest when you need it and stash it out of the way of your elbow when shifting more enthusiastically. Totally worth the price for me.
Here are some pics to show how it works.
Angled shot with armrest in low position:
Angled shot with armrest in high position:
Side shot with armrest in low position:
Side shot with armrest in high position: