How does the "Invisible Chair" trick work?

I’ve seen quite a few videos and photos of people appearing to recline or sit in an invisible chair with no visble means of support (all the photos and videos seem to show them pivoting on one leg, but it still seems like an improbable balancing act).

I’ve know people who swear blind that they’ve seen it done in real life and they aren’t just photoshopped, so are they telling the truth or just BSing?

I’ve seen pictures at least of people on a pedestal reading a newspaper at an airport. You don’t see the supporting metal bar hidden up their pant leg that leads to a chair in their pants they are sitting on.

http://illusion.scene360.com/video/8990/a-man-sits-on-an-invisible-chair-at-the-airport/
http://www.youtube.com/watch?v=1h-OZOLjY54

The shoe is nailed into the ground, there’s a strong metal tube going up their leg inside the trousers at an angle to the ground and a metal plate inside their trousers that they sit on.