Standing 2BHS. On tumbletrak or 4-inch mats first, then transfer it to the floor. IMO, the best way to train ROBHS is to train standing 2BHS first. And this may seem counterintuitive, but I think you should NOT try to make the connection seem like a rebound.
I did a clinic on back tumbling a few years back at Woodward where I talked about this in more detail:
About 3:30 is where I start talking about building the connection, and 4:20 (lol) is where we show the drills and progressions I like to use. Hopefully it all makes sense, but if you have any further questions or if I didn't explain anything well enough in the video, feel free to ask!