I truly don't understand the physics of fly line backlashes

What backing is that? I don't like loose braided backing like that. I think it has too much "squish" factor and leads to mono or other running line digging down into it. I had one last year that was quite the head scratcher. No bueno!
It's from a spool of Rio standard backing I've had for years. I think you're on to something though. There's a lot of posts saying the backing is loose/twisted, but this reel has never touched a fish, and most certainly never had the backing off the reel since being filled up. It's been used a handful of times to cast a sinking line to lockjaw coho here in my local waters, with all of maybe 60' of line pulled off at the most. So the squishy-ness of the backing may be at least part of the culprit. Not sure how else my running line could be that buried.

Good news is it's full of gelspun now. No more squishy backing.
 
I have a habit (probably a bad habit) of spinning the spool up with line hanging below the reel (no tension), usually I'm paying attention and it doesn't become an issue but more than once I've spent quite a bit of time lake or creekside fixing a "crossover backlash f-bomb mess".
Done that exact thing for years , spin the fly line back on the reel , unless there is too much then it's stripping , but most times I spin back on the reel to handle the fish . So far I have been lucky I guess never have had a backlash on a fly line . Many times that line thats been spun back ,got stripped back out by the fish . Been a few times stripping back the fly line in my apron , got it tangled up to where it took me awhile to get it untangled ,amazing how bad it can get . Now I have done it , probably out on Henry's tomorrow I'll find out what a backlash is .
 
I have dealt with it a handful of times. I have always questioned how did this shit-mess happen? I’m careful about retrieving my line to the reel. Sometimes I can get it resolved quickly and other times I have had to take the spool off and fart around with it for double digit minutes and just deal with it.

Regarding swivel? You attach it (between) to connect backing to fly line? I have read about some people using swivels but always thought it was used on the leader connection. ????
 
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