I think you're thinking of Wimblemimble and ZK-snarks. They use cryptographic techniques where the signatures need to "add up" to what they should in order to be valid -- demonstrating that no new tokens have been added in the course of the new block.
Quite cool approaches. The problem is that you can't attach data to transactions, so only useful for a subset of applications, those unlikely to create much bloat in the first place.
