The ffw system uses a special freewheel with (stiffly) individually moving cogs. The advantage is as described: you can shift if the wheels are turning, but you don't need to be pedaling. For whatever reason, it was only ever implemented in a low-end way. This was often paired with a Shimano Positron rear derailleur, but that's not a necessary component. My experience is that the front freewheel is loud as hell--like three times as loud as a regular freewheel. The chainring spins disconcertingly. The system is technically interesting, but not particularly better than a regular freewheel, particularly as implemented. There's a good article about it on sheldon brown.

I've seen this implemented on one-piece bottom brackets and the (slightly nicer) three piece. Imagine a normal bike ride with an additional ZZZZZIIIIIIIIIIIZZZZZZZZZZZZZ