Try clicking rapidly 7 times while the light is off.

My understanding is the ROT66 has 2 lights on the button that are always on, and 2 lights that are controlled by a pin on the controller. The latest build of the Anduril user interface has as default behavior to change the setting on that pin in response to 7 clicks.

If you have one of the earlier ROT66 with the Narsil user interface, I will have to defer to someone else, as I’m not well versed on Narsil.