Deere 3rd function joystick - what does that front single button do?

Does anyone know what the lone button on the front of the up/down buttons on the 3rd line control setup?? Is it so just to pretend you're launching the AIM-9's??

It was suggested today that there may be a JD factory way to make it a on/off for a continuous flow setup.

Any ideas?

-J.

Attached Images

• Image may be NSFW.
  Clik here to view. Untitled.jpg (23.1 KB)