thats why he talked about how he has to work out a lot etc so he can do the things he does...if he didnt work out and did the building walk yeah he would be bending back but he's strong enough to keep his back straight, make sense? its called muscle...he does like 1000 to 1200 sit ups,crunches stomach excersises a day, im pretty sure he has the strength do be able to keep himself straight like that, just like the people that can hold a pole and stick out horizontal from it, its amazing what muscle can do
That still doesn't explain the necklaces/chains not falling back toward the ground.
Notice in the video, they show him putting one foot up on the wall...they shoot to an interview...they go back to Criss and he's already 6 or so feet off the ground.
It is so obvious that it's camera work that I am completely blown away by the fact that I'm even having this debate with a fellow student of magic!