What do you make of this behavior?

This is what it ALWAYS looks like when they play their little games. I didn't intervene until Nigredo came up with that dangerous look in his eye I've mentioned a bazillion times.

His ears flatten out, his posture stiffens, and he gets an odd look in his eye. I saw it immediately (about 0:32 you can see it) and went to distract them so things didn't get ugly, which they sometimes do.

He's only playing but that look means "I'm going to PUPPY you!! Like, RIGHT... NOW!!" and he usually charges or does something extremely pushy or annoying (like he did in this video) to scare Bunny and it always ends badly. Once he reared up like a horse and Jade instantly flattened to the ground, yelled (out of fear, he did not even touch her), and wee'd herself. Poor girl

I wish they could play nicely, but this is really what usually happens if they try and play with each other. A stiff, kinda pushy, kinda weird game. In the past, failing to distract Nigredo in time has caused Jade to be knocked over, or scared into weeing or yelping, random items to be knocked over, cats to be trampled, and other bad things.

I don't think Jade would ever bite him, she isn't that bold. Nigredo is not aggressive in any way and would never bite anybody or anything. I'm not worried about a fight. It's just that Jade is old and she gets scared and wees and whatnot when he goes overboard.

I realize that they are WAY different ages and sizes and that Nigredo needs a dog his size and vigor to play with, but I suppose it's merely wishful thinking that they can ever play nicely without it seeming forced or just not even play but some kind of ritual or something.

He played with the dogs in the dog park normally and it was fine, but Jade doesn't like it and he doesn't realize it, or perhaps doesn't care. I don't immediately tell both dogs to sit down, because I want them to be able to interact, but it never works out.

Nigredo probably has deep down in his genes the 'Pack leader' thing, he is just trying to make sure Jade will be the 'junior' in the pack, as you say that never involves real harm but can involve a bit of 'discipline' even as in this case by way of 'play'

The 'age' thing is not really relevant to dogs, what do they know about old age? they just live 'till they die, it's a simple philiosphy that us humans don't have, we always have to complicate things by trying to bring in our knowledge from books etc.

I agree with you it's best not to let things get too excited because that's when accidental injuries might occur. Let them play and then do what you did and seperate them. Don't chastise either of them, comfort Jade (as you did) doesn't look to me like there's any real problem here.

Thanks. Nigredo can be a tad stubborn sometimes concerning Jade. It took me a few seconds to distract him, but Jade gladly welcomed the distraction. But, as you can see... it takes less than a minute for things to deteriorate, so at times I'm thinking I should tell them both no before they even begin these shenanigans.

No, it's not like you've got two humans there, dogs are different, they need to interact with each other, they don't have a sophisticated means of communication, they don't speak English or any of the languages we use, they have to play, growl, and give off other signals to let the other dog know what's going on in their mind.

As I said before I don't think you've got a problem there, when we had two dogs there would come a time when it looked like they were getting too excited, then I would try to calm things down, but they were much worse than in your video.