I have been playing with some of this guys work. \
Like respect, flexibility is earned. It is earned through the proper strength, on fundamentally clean motor patterns. Flexibility without strength is not all that common. But we spend so much time stretching and forcing length, and this is a flawed path that will not get you far or to places you truly want to be.  There are many methods, but slow clean movements are important in the early stages, just like in this video. I like to use the 9 second rule in most reteaching in my clinic, 4 seconds to get to the desired end movement, 1 second to hold the end skill, and 4 seconds to get back out of the skill.  Modify as you like and as necessary. Speed can cheat.  Remember, skill first, then endurance and repeatability on the skill, then strength on the endurance based skill.  S.E.S !
This is a Beautiful video, beautiful movements, amazing strength and flexibility.  Things to strive for (and it can’t hurt my Jiu Jitsu either :)  -Dr. Allen