It goes something like this: 1) Perform some reversible operation 2) Copy answer to another location (At some cost in heat) 3) Perform reverse operation, whi

It goes something like this: 1) Perform some reversible operation 2) Copy answer to another location (At some cost in heat) 3) Perform reverse operation, which rolls back memory to starting point When you're done you have zeroed out memory, and the only costly operation was the copying of the answer.