It can, for example, let users prove that they paid the taxes due on all transactions, without revealing those transactions, their amounts, or even the amount of taxes paid. As long as the policy can be specified by efficient “nondeterministic” computation, it can (in principle) be enforced using zk-SNARKs and added to Zerocash.

They describe some more interesting properties of their system here: http://zerocash-project.org/q_and_a#what-about-the-balance-between-accountability-and-privacy