How a Rattleback Works
A Rattleback or celt has a very specific shape which means that it will spin in one direction only. When spun in the non-preferred direction, the Rattleback's spin will slow and it will begin to rock from end to end. This rocking motion is then transferred into spin going in the preferred direction. In short, spin it the wrong way, and it will stop and spin the other way! Originally made from wood with decorative carvings and patterns, Rattlebacks are now commonly available as plain resin forms.
To understand how a Rattleback works we need to take a look at its shape.
The Rattleback is flat on top and has an asymmetrical ellypsoidal bottom. The specific alignment of the long axis of the ellipsoid in relation to the long axis of the flat top causes the directional preference. In other words, they're not parallel so the rattleback is predisposed to spinning in one direction.