I have being going over the theory behind the output gap and here is an explanation – written a few years ago. Probably not so applicable to the economic environment today
Just as Messrs Friedman and Phelps had predicted, the level of inflation associated with a given level of unemployment rose through the 1970s, and policymakers had to abandon the Phillips curve. Today there is a broad consensus that monetary policy should focus on holding down inflation. But this does not mean, as is often claimed, that central banks are “inflation nutters”, cruelly indifferent towards unemployment.
If there is no long-term trade-off, low inflation does not permanently choke growth. Moreover, by keeping inflation low and stable, a central bank, in effect, stabilises output and jobs. In the graph below the straight line represents the growth in output that the economy can sustain over the long run; the wavy line represents actual output. When the economy is producing below potential (ie, unemployment is above the NAIRU), at point A, inflation will fall until the “output gap” is eliminated. When output is above potential, at point B, inflation will rise for as long as demand is above capacity. If inflation is falling (point A), then a central bank will cut interest rates, helping to boost growth in output and jobs; when inflation is rising (point B), it will raise interest rates, dampening down growth. Thus if monetary policy focuses on keeping inflation low and stable, it will automatically help to stabilise employment and growth.