Nice demo Daniel. But something OT distracted me, especially when the horse was laying down with his hind foot so close... what is the purpose of that back cinch, when it is so loose??
I've only seen a horse once with a hind foot caught in a too loose cinch, which happened when the guy lay the horse down, but it wasn't pretty & I couldn't help thinking it was likely in this case when the horse was laying down saddled.
I'd say that girth was just right. You are correct about what can happen, but it's a fairly freak deal, especially with a horse that knows what it's doing. I haven't had that happen, but I did once have a colt get a hind foot through a stirrup. Don't ask me how he did it, but he sure tore that saddle up.
Truly, I could have done everything with no back girth at all, so it really served no purpose. I do, however always ride with one because I mostly start colts and ride problem horses. A horse which isn't used to a back girth getting accidentally introduced to one can be a helluva wreck as well, so mine get used to one.
If you'll look right when he's laying down, the back of the saddle rises up quite a bit, making the fairly lose back girth pretty well snug on his belly. People who do ride with a tight back girth frequently don't realize just how much of your horse's ability and mobility you compromise. It is like strapping a rigid backpack to your back. You can't bend anymore. I knew that there would be a few fairly hard stops in this ride, which would cause the back of the saddle to rise up, and the laying down, again, causing saddle to rise up, so I gave him some slack. NBD to me. Adjust yours as you see fit.