Judging by my experience of trying to swim front crawl with an ankle band, I would be apprehensive about trying it with backstroke, However you never know, so I will give it a go.
In front crawl I sometimes fishtail a bit at the beginning of the length, which perhaps is due to a somewhat lopsided push off and first pull, but I think the fishtailing then disappears. I have a very non-propulsive kick and swim freestyle with a small two-beat or sometimes four-beat kick. Backstroke is with a flutter kick and I although I do move without an arm stroke it is very slowly.
I can usually swim faster with the old double arm stroke with breaststroke kick, I have just learnt the German term for it: Rückengleichschlag, and was amused to discover that it it is also known as the Old German Backstroke, because in Britain it is known as the Old English style. I wonder what the other European countries call it?
It is very easy to do and I used to see old people doing it when I was a boy sixty odd years ago, so I didn't really have to learn it.