This mode replaces the colored vignette with a cubemap-base skybox cage anchored to the Motion Transform. This is a performant way to give the user a static reference frame instead of just fade out the "real" world.
TIP: VRTP includes starter cage cubemaps in VrTunnellingPro/Prefabs/Skyboxes. By default these import at low res. For full quality, switch to 4096.