It's not documented directly since it's done with Steam's built-in controller customisation. Under desktop mode in Steam Settings -> Controller -> Desktop Layout -> Edit Layout – there are two Action Sets with the start button bound to switch between them on long press. You'll hear a sound when it happens as well and the game will detect it as an Xbox controller.

This acts system wide as long as Steam is running but it won't give you the per-application customisation you get by adding it to Steam.