Do you have a link of somesort that explains his posture and head over knees? Thats an interesting view... i feel he's similar style cutting as Kovalev, and his body frame looks to probably be about the same as kovy too - the Russian bear. I cant recall Kovys straight speed, always found his speed came from cutting..
Kovalev was much more upright and was extremely strong. The problem with Demidov's posture is that it makes it more difficult to alternate his speed while carrying the puck and limits his ability to pull up and deter defenders from closing out on him on the rush. I find in watching him he is almost compelled to bring the puck into the teeth of the defence at times and this will be something of an issue when the defenders become much more competent at higher levels.
His posture should be closer to sitting in a chair with his torso in a more balanced space that allows an easier access to options that are in a 360 degrees spectrum. This is true for McDavid, McKinnon, Kucherov, Pastrnak, Eichel, Mathews, Bedard etc.
He basically bends too much from the hips and not enough from the knees which forces his upper body forward and over his knees which limits speed, explosion, and agility while in full stride. He is fine when he is not in full stride but for him to realize his full potential he will need to find a more balanced and athletic posture.
Don't get me wrong, I absolutely love his game but when asked to find an issue this is by far the most obvious one. Despite his flaw in his stride/posture he is still very dynamic on the rush which is quite amazing as his edges are sublime and he uses them in a very unique way to manoeuvre and regulate his pace which I am sure can be surprising to defenders, especially inexperienced one's in the MHL. I would just like to see him a little more upright before coming to the NHL and attempting to translate his game to the NHL against the best defenders in the world.
I don't have a link but I am sure if you google skating posture you will find more information on this. You may end up with a lot of figure skating based explanations but I am sure that there is a lot of information on this. Having been around the game for close to 50 years these concepts are pretty ingrained in my mind so I don't actively search for such sites very often but I love the fact that you are interested in this which is something that all of the fake scouting sites should be doing as well as they are almost always clueless about skating.