The NIP (Nostr Improvement Proposal) defines a way to generate and interpret Proof of Work for nostr notes. The difficulty is defined as the number of leading zero bits in the `NIP-01` id. To mine a note, the second entry to the nonce tag should be updated, and then the id should be recalculated. This ensures that the miner has found a valid solution. Clients can protect against bulk spammers by requiring a target difficulty commitment from miners. This means that miners must agree on a specific level of difficulty before they start mining. PoW (Proof of Work) can also be outsourced to PoW providers for delegation. This allows users to delegate their mining power to others who will mine on their behalf.