Ha I've done that a thousand times in the past. Now I just use a grunge brush and rotate the wheel by hand. Doing it using engine power just makes a bigger mess IMHO. As far as losing fingers, well if you put the rag on the sprocket or on the chain as it LEAVES the sprocket...
To be honest, the best way to do this is with a grunge brush.
and chain cleaner. I bet this guy changes v belts on a car while it's running.