[pas]

There was a paper a few years ago (when the need for proper anonymous bitcoin spending came up), something about zero-knowledge proofs, and you can switch your known bitcoin (public key, basically) to this anoncoin and you can then switch that back to a bitcoin, but all outside observers will just see that your known address "bought" an anoncoin. Of course, if someone observes the whole anoncoin switcher network, all they would see is that anoncoin users together bought some bitcoins.

But here is the relevant wiki article: https://en.wikipedia.org/wiki/Zerocoin