@Partway Mist  
I cannot imagine a different stretch that would end with the foot (cannon) behind the back like that. The only thing that somewhat resembles that is this:
 

 
but the foot is in front of the torso, so I got nothing.

@ymom2  
Ah yeah, I’m getting what you’re saying. I was just focusing too much on the extremely short calf and making it work than the rotation of the hip joint.
 
I certainly agree with you on the fact that the leg behind head pose is more in the hip than the knee (since the knee only bends in one direction). However, I’d also imagine that the pose focuses just as much on the bend of the back, which is also what I was forgetting in the image. Those alone would probably improve the pose tenfold. I’ll try and work on the knee joint, to improve it to the point where it looks good in any position, but I’ve really got to put more focus on the rest of the leg.
 
That diagram of the positions of the bones was a bit cluttered, but reading through everything else, I can get the gist of what you were trying to show. Pretty much get the hip pointing backwards and away, and try to get the calf behind her back. The latter would be difficult, because of how small I make them (it’s pretty much entirely contained within the oval of her thigh), but I imagine that so long as it’s positioned correctly it would help make things more believable.
 
But I’d have to disagree with you on one thing: the hip-opening exercise is not the same as the leg behind head pose. Granted, putting ones leg behind their head is an exercise for hip-opening, but that rotates the leg into the body, whereas the hip-opening exercise I was trying to explain rotates the leg out of it. Imagine it like the straddle splits, but lifted up at a higher angle relative to her body, and her foot bent behind her back.
 
Feel free to let me know if I’m missing any details. This is really helping me out.

@ymom2  
I see what you were saying, in your picture you were twisting the knee joint not the hip joint. The knee joint cannot physically be twisted. The hip joint is the one that twists in order to rotate the leg.

@ymom2  
Also, I forgot to add that for the “behind the head” pose this is a better image. In the real image the knee rotates outward and the heel rotates inward so that it can be placed behind the head.
 

 
If done this way the heel (top of cannon) will be behind her shoulder blades and her knee will be jutting to her left.
 
I practice this kind of stretching and am kind of obsessed with it. I can get the heel of my foot to touch my shoulder blade but can’t get it quite as far down as the real picture.
 
I hope this inspires you or others to make more of these!

@Partway Mist  
The “hip opening pose” shown and “leg behind head pose” are the same thing.

@Partway Mist  
Maybe this will help.  
 
In the image of a real contortionist the femur, knee, shin, and foot is behind the back. In your image the hoof (toes), and cannon bone (foot bone) is behind the back. The tibia and fibula bones(shin) are somewhat merged into the knee bone in your sketch. The knee itself seems to be in front of pinkie.
 

 
This is kind of how this contortion would look like if Rarity kept lifting her hoof behind her shoulder blades. The femur, knee, cannon, and hoof would be behind her back.
 

@ymom2  
Okay, I had a go at it, and it didn’t really turn out so well:
 

 
I’m not sure if it’s the anatomy, or if I’m just doing something wrong, but I don’t think I can make it work.
 
I’ll probably return to it in the future, once if I redesign how I draw hind legs or improve on hind leg posing, but for now I’ll stick to the hip-opening stretch when I draw this sort of pose.

@ymom2  
Hey, it’s the artist here.
 
Yeah, I’m very much aware of how it differs from the normal “Leg behind head” pose, however I was going more for a similar look rather than a similar bend in this instance.
 

 
This would be a better angle of the same sort of pose, albeit a less extreme version. It should be more obvious that it’s more of a hip-opening exercise here.
 
Drawing a proper “Leg behind head” pose may not turn out as well as this, but it might be a nice exercise for me to try. I’ll have a go at it, and share my results.

The top right picture is a little wrong, the knee should be behind the back.
 

 
Imagine like this picture except the knee is pointing outward so that the hoof can slide down the back.

YES

I find this really cute!