Why does Ariadne's totem work?

It follows the same concept as Arthur's special dice, basically the exact same concept.

Consider for example, how Arthur would know the die is his. He couldn't just rely on which face the die landed on. For one thing, a loaded die doesn't always land on the same side. And then if it did, and that was his check for the totem, it would just take observation to be able to fake his totem and trick him in a dream. What Arthur would know and be checking for when he picks up his die is the weight distribution. He had to become intimately familiar with the weight and feel of his particular die, and this allows him to know that it is his.

Ariadne essentially did the same thing. She machined her bishop to have a specific weight distribution that isn't apparent just from observation. She knows what the weight distribution of the bishop would feel like (and can test it by knocking the bishop down, similar to how Arthur would know the feeling of throwing his loaded die).

It's not about how easy it is to knock over, it's a combination of how easy it knocks over, how it feels when knocking it over, which direction it goes, etc. Someone just observing the bishop fall might be able to come close to reproducing the weight distribution; but because they don't know how much force she applies when handling the piece, they can never perfectly replicate it if they don't pick it up themselves. This is why the rule was to never let anyone else touch your totem. They need to be sure they are the only ones who know the specifics of the totem.

Even Mal's top would have had it's own weight distribution presumably. A top spinning on it's own isn't a very secure totem and could easily be reproduced by someone else's dream. They wouldn't know the weight distribution though, and that combined with never letting anyone else touch it is one of the few ways you could ensure someone doesn't dream a copy of it through observation.