Giving some "behind the scenes" look to it:
The RR2 fan control looks like a "dimmer" to myServer.
myServer's dynamically created Lighting widget sees the fan device as a dimmer so displays a slider to control the "light" intensity.
But the RR2 fan / myServer driver is creating variables on the status the RR2 sends back.
And the myServer RR2 driver can send a command to the Fan to turn on / off or go to a dim value (Low speed).
So, using myDesigner, one can create a button that displays the RR2 variable value (on / off / 50%) to highlite or not
And that same button can trigger a Lighting family command to send a Lighting|1~Level~50 to turn the Fan on "Low"
You could use the slider to figure out "is 50 a Low Fan speed value?" Move the slider, see how fast the fan is moving, look at the current Var value, and use that value in your custom button. I am betting that the RR2 Fan sees a range of dim values and goes to the speed that is closest to that Dim value. Once it hits that speed, it then reports back it's current actual dim position. That's the value you should use. And that's why the dynamic slider moves to a new position close to where you placed it once the RR2 device reports back it's actual state.