This looks very interesting, but as I'm poking around on there I want to share a word of caution... 🙊
Yes, I believe that every one can get flexible with practice (my yoga got me into poses I couldn't even do as a kid), *BUT* it takes time and caution. Don't forget that most contortionists/gymnasts do this since they were toddlers (many, many years), and I'm sure it goes much easier and further when done while bones & joints are still growing (not sure if that automatically means healthy 🤔 )
These exercises look very extreme and they don't really talk about how to get in there *safely* (or what to do if you don't even get into the starting position). Even the warm up is something I would never, ever try without being already warmed up - in yoga class most of this comes in the middle.
This whole thing reads more like it is for people who are already quite flexi and have some basic training on how to do this and how to avoid injuries...
In general, I've learned to resist and control every move/pose by engaging your muscles and make sure the joints stay integrated, rather than just sinking into your flexibility so that you don't injure your tendons, ligaments and joints. Once a tendon is overstretched, you won't get it back without surgery and you risk your joints dislocating by themselves. So just pulling/squeezing yourself in position sounds very dangerous to me.
For the backbends, my yoga teachers explicitly warn to do a backbend from a few vertebrae only - the already most flexible one will get more and more flexible up to the point of a serious back injury. We learn to distribute the bend over the full range of the spine, which means a lot of chest opening first before even thinking about full backbends.
They don't talk about hip flexors at all, which are important for both splits and backbends.
Enough eyebrow raising - but please stay save and listen to your body!! 🙊 🙊 🙊
