Bitcoin payments are processed by a network of semi-independent auditors known as miners. Each miner receives a subsidy proportional its share of the network‘s total computational power, or hash rate. This reward system has driven intense competition among miners, who collectively double the network’s hash rate every month.
Miners contributing less than half of the network hash rate compete under the same set of rules, but a miner contributing a majority of the network hash rate gains unique powers. Most discussions of these powers revolve around the potential to selectively rewrite the transaction history and prevent transaction confirmation.
However, a majority miner can play a different game with greater profit potential and lower risk.