I saw the shorter version and it seemed suspicious because of the tension in her hind legs. But I was not sure what exactly was going on there. But I noticed the chain too. And the fact that it came from z18x. Thanks for reporting it.
Yes, unfortunately the shorter version itself didn't show too much "direct" aspects, I was quite unsure about that one as well. Full length one didn't seem to include this exact short clip, as the ends were different, so she was probably copulating multiple times.
Longer version shows lots of body signs of discomfort, cramping and closed hind legs, scraping with the hind legs on the ground, trying to walk with the front legs on the lower bar of the arm rest while the upper bar stopped her, her body / hair / short snout looked either like a "permanent puppy" looking breed or (more realistic) like a dog between 7 and 14 months of age.
Young dogs in first heat in possession of backdoor breeders in east Europe were on videos (as someone in the leader team of shelters to that time I saw those) almost the same.
They reacted comparable and often tried to run away the second a male dog actually mounted them, even if their hormonal balance let them show interest and even lifting before. They panicked due to pain or discomfort, as the first heat is the -first- time in the body structure of a female dog where her reproductive organs actually -begin to widen- enough for copulation with similar sized dogs. That's not even nearly grown into adult composure, but it is at least survivable without internal damage. ?
And the longer video showed comparable signs.
Even this aside, as I can't prove it, what was clearly visible on one part of the longer video: The chain was hold by the second participant leading around the arm rest in direct front of the female dogs head, while that arm rest was quite high and in the way. This speaks for "hold in place", which ruins any consent. Nobody needs to hold a small female dog with a chain around the neck and quite roughly including holding in stance by the tail and back legs / belly if she consents.