Sorry about the stupid advertisement, but it is definetly worth the wait. What happens is that 10 seconds after someone enters the Port-O-Potty, the roof flies open and the toilet is lifted up out of it, so everyone can see them taking a shit or whatever.
http://www.metacafe.com/watch/165925/crazy_japanese_port_o_potty_prank/
http://www.metacafe.com/watch/165925/crazy_japanese_port_o_potty_prank/