I'm not familiar with a "flip mirror".
It's just a generic term which can cover a range of technologies. It's a good search term to start with. My telescope has one internally for astrophotography: I can leave the camera attached at the end, set up the shot with the top eyepiece, and flip the mirror out of the light path for the actual shot. This is front-silvered stuff, of course, so the image degradation is minimal. I know there are eyepiece extensions out there and telescope designs that allow "rear look" and "side look" on the same telescope. I was just trying to demonstrate that they're not mutually exclusive.
And of course, since I'm into astrophotography, I can navigate guru-like through a right-angled lens and an inverted image. But my brain would explode if I tried to follow an aircraft with this setup. Hand-held, I'd probably fall over, too.