Enrique, The knob switch is not a switch assigned to the mix. It is the adjustment knob that is used to alter all the settings depending on what menu item you are in.
Once the flap/elevator item was entered & the jet taken off, the knob would act like a trim to adjust the mix. once the adjustment was set, a single press on the knob locks the setting. I have had some scary moments trying to land jets when the mix was out & thought it would be good if the mix could be altered in flight at a safe height.

Regards