Consider the case of missing disinflation during the GFC. Theoretically, as firms still operate below their full capacity, when demand is gradually stimulated, they will start producing more without incentives to raise prices yet; firms will increase
prices only when demand gets sufficiently large that it approaches their capacity constraint. On the other hand, during boom times, a small amount of additional demand will translate into strong price increases. In other words, the Phillips curve could be almost flat when the output gap is very negative, approximately linear when the output gap is in the
neighborhood of zero, and becomes convex when output considerably exceeds potential output