What is the political business cycle and how does it relate to whether the central bank should have discretion or use a rule?

The political business cycle describes the idea that politicians may manipulate the economy to serve their own political ends. For example, the political party in power might want to generate an economic boom prior to an election, even if this policy is ultimately not in the best interest of the country. If the central bank had to follow a policy rule it would be unable to manipulate monetary policy for political gain.