Mixed Reality - is there an "on" settings?

VicoDrive
Settler

Mixed Reality - is there an "on" settings?

Ive been checking out the mixed reality ability that Vive can do.  Ive seen many tutorials  and How-To's on it and have a pretty good idea on how everything works.  The only thing that every tutorial seems to leave out that I have seen is how do you actually start it?  Is there an "on" switch in the game or Steam settings that turns the camera on or mixes the VR and reality together once you have everything else set up?  This seems to be the only thing I am missing when attempting to do this.