As far as flywheel hubs go, the key is just an indexing tool. It's the taper of the outside of the crankshaft and the inside of the flywheel hub that actually grip each other (Which is why it's so important to lap the hub to the crank).
So think of it this way--cut a flywheel key in half, long-ways. now put it back together, but one half offset to the other, meaning you aren't quite lining up the pieces. The offset key works the same, but is more accurate.
Don't know if that makes sense, but there you have it.
yes this makes complete sense. Appreciate it guys