Tumbling Rings Trick
Whilst holding the top ring A with one hand, grab one of the rings B or C with the other and release A. If you have grabbed the correct ring in the correct place, i.e. front or back, the the top ring will appear to tumble to the bottom of the chain.
In the picture the only position that would achieve the effect is the front of ring B. A little experimentation will produce an accurate presentation of the trick every time.
To make the toy google "tumbling rings of antiquity" or use https://www.youtube.com/watch?v=9JbeQyk_rFQ
To source the rings google "split rings" or use http://www.ebay.co.uk/itm/Split-Key-Rings-25mm-Nickel-Hoop-Ring-25-50-100-and-200-/281265162339