I too want to learn the horse stance but everyone says the same thing and then no one will tell in full detail.
The horse stance is literally relax all, extend both glutes for 20 or 30 frames then hold all.
You're a 4th dan, why are you asking such a question, you're only moving 2 joints then holding.
Even if i agree with NotNoob and Banappeal, this argument is stupid, and doesn't belong in the beginner sanctuary. Holy shit guys.