The sync flag would consist of a hash of the previous block, a Bitcoin address, and a nonce, and would propagate through the network quickly. However, concerns were raised about the potential for selfish mining of flags by big miners who could withhold them until the advantage gained from doing so is less than the mining reward. This could lead to a loss of decentralization features, as collusion would be encouraged and in-groups would gain a massive advantage in main-chain mining. One commenter suggested that sending out a block header without any forks could achieve the same result as the sync flag proposal.