[bjg]

The point of the totem is that only you know how it behaves, and how it feels and how it's weight is distributed.

If someone else has held and seen your totem, they can possible recreate it exactly in a dream. If they can recreate it then they can trick you into thinking you aren't dreaming when you actually are.

[AngryParsley]

The totems seem to be pretty vulnerable to trickery, but the film didn't explore that much. For example, (spoiler alert) replace Joseph Gordon-Levitt's loaded die with a fair one. He'll think he's in a dream and kill himself. Making an ever-spinning top would be harder though.