Is it physically impossible to back in a motorcycle without a back brake and/or a gear downchange?
Depends, on a two stroke I would say its impossible, on a four stroke v twin with no slipper clutch, yes why not? Specifically whats the difference between shutting the throttle with the engine at redline and downchanging? In each case the force the engine will apply to the back wheel is similar if not the same. Unless you mean a botched up downchange but thats more poor technique, something I have done, not controlled backing in. Whether its the engine applying the force through the chain or the rear brake, the rear wheel and tyre do not differentiate the difference like some sentient being, sliding only when its being done 'correctly'. Either the force is enough to send the wheel sideways on the lean angle or not.
I dont think its impossible, rather a much harder to do, extreme speed, expert rider type of technique. Watching those stunt dudes do their thing, what with the mx backflips etc if someone described on the internet they could do that a few years ago I would probably call ........ without seeing it with my own eyes, yet these days even a backflip on a bike is a fairly common occurance.
[media]http://www.youtube.com/watch?v=20XsaHpRQC8"]http://www.youtube.com/watch?v=20XsaHpRQC8[/media]