I have a black Silkier Rooster who does the same thing. His condition has gone on for quite a long time... Perhaps more than two years now! As time has gone on, he is getting better at controlling his muscles. It is really only from the base of his neck up, that he has trouble. He sleeps, as you describe, with his head between his feet, and his butt sticking up. He kind of wedges himself in a corner to sleep, I guess so that he stays balanced. Anyway, when it first atarted, it was suggested to me that he might have gotten struck on top of his head, perhaps by a beak. I understand that silkies, polish, and other poofy-headed chickens, often have a thin, or partly absent,scull on top. This makes it much easier for them to get injuries to their brains. This makes sense, because "Fuzzy" the rooster used to get bullied a lot.
He now lives in my bedroom, in a wooden box, rather like a bassinet. He has good days and bad days. On the bad days, I help him to get situated so that he can eat and drink. He has to really focus and relax to get his neck ti stop whipping around. Like people who stutter, for instance, the more excited or agitated he is, the more pronounced his difficulty becomes. I just put my hands oñ either side of him, lime bumpers, to guide him and help him stay steady.
The aspect of his troubl3 that it took a while to recognize for me, is that sometimes he would get stuck head down while drinking. Not good, of course. I solved this by getting him a very heavy square vase, that is tall enough that he can't get his head very far into it. Certainly not far enough 5o tumble in. It has to be kept full, or he can't reach to drink
I have a friend who says that I am cruel not to have him put to sleep. But fuzzy fights for life. I have seen enough chickens just give up, that I know the difference! He wants to live, so I help him as much as I can.
8 have no idea whether your own breakdancing chicken has the same condition. But, if so, maybe this info will help. All the best to you both!!!
Patience