I, too, have been curious about how this works because I recently came across one of those Brampton flip-flop fixed/free (5 spd??) hubs. It's technically up for sale (see signature) but I'm still debating whether I have a use for it. Thus far I haven't felt the need to have the same bike function as both a fixed gear and a multi-speed...
Apparently back in the day it was common practice to switch over to fixed for winter training with these hubs, but it's kind of a commitment, since with a 5-speed it wouldn't be a matter of just flipping the wheel over...
I like to always keep a fixed gear commuter among my (currently two) bikes.