How does Captain America's Shield stay on his back?

[cmorin543]

so... I was looking through some pics on google of cap to get a close up of the leather strapping used on the holster for the shield and came across this video from the filming of Avengers 2 and it shows the film crew putting on and taking off the shield from Chris' back. https://www.youtube.com/watch?v=fgm1Q7EVpGE