Flopping onto his back-what does it mean?

Well its day 1 of the bonding. Theyr in separate pens saying hello thru th mesh. Ruby keeps wandering off but Beau is totally obsessed. When they meet they nip each others noses thru th mesh,n wen Ruby goes away,Beau stamps his feet,n if she doesnt come back he throws himself over onto his back,like a happy flop but its all the way over,and right up agenst their shared wall. Is this like a submissive thing?

From my own experience with rescue buns, rabbits react very differently through the wires than they do when actually placed on neutral territory.

I always bond rabbits over a 48 hour period as I find this works better and is less stressful for the buns.