Proof-of-Fraud is one of the key features of Bitcoin-NG. It is used to prevent malicious forking behavior and this consequently prevents double spending. In theory, if the previous leader tried to double spend in one of the micro blocks during its generation, the current leader can submit a Proof-of-Fraud within one of its micro blocks and receive 5% of the original coinbase reward for the proof. Then in return, the malicious leader would not receive their mining reward. The rest of the coinbase reward and fees become locked into a special lock account.