I press it in to activate them.
I press it in a second time to deactivate them.
This is for dive flaps though, not the regular flaps.
The light on the throttle base changes color with each click, so it's probably a mode selector for the throttle.
For regular flaps, I use two of the buttons on the actual throttle; the two under my index and middle fingers. The ring finger activates WEP. This is how I had it when I first got CH stuff back in 1995, so it's familiar to me.