Do you mean how people tie the ring with a string and hold it over the [person] is what I've heard of not for ducklings. Not sure how it's done though. want to explain.
I think this is what your talking about... http://www.ehow.com/how_6704388_using-ring-determine-baby_s-sex.html
LOL, i know of what your meaning. We tried it for fun, because we were doing it with my daughters silkies. It was mixed, it guessed wrong with 1 of the 4, it said 3 girls, 1 drake. It was 2 girls, 1 drake.
I'll have to ask my kid what the results were for the silkies, i don't actually recall. Anyways, it's strictly for fun, i wouldn't stake the farm on it.