Basicly you are wearing a wireless suit **** your movements, so when you walk the character in the game walk. It will copy your movements. And by wearing a special wireless helmet you will have something similar to a "first person" view. To avoid you from moving into stuffs in the real world you will be pulled up in the air, hanging in something similar to bungy jump ropes to enable all kind of movements :P
Knowing that they use some kind of suits for animation I guess this idea isn't so far from reality
(Maybe there actually are prototypes of this out there O.O)Making a mmorpg for this system would be awesome, think about the PvP >:D
I wish I can play games like that. That will be cool, but you will need a lot of space for something like that 