You apply power to the camera. As soon as the head unit detects the camera signal, the display will switch to the camera view. When you don't want the camera view any more, either turn its power off or, once it comes up, there is a touch-screen button to dismiss it and go back to the map view (and once you've done that, and if the camera's power is still on, there will be a button on the map to switch back to the camera).
This is why you want to wire the power to the running lights... If you're traveling down the road in daylight, to see what's behind you just flick your lights to the running light position and then back off once you've seen what you wanted to see.
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam