Krugman proposes an answer:
What I want to do in this note is sketch out a New Keynesian-type analysis of the role of fiscal policy in a liquidity trap. The bottom line is quite striking: aside from some qualifications I’ll discuss at the end, when the economy is in a liquidity trap [and monetary policy is up against the zero bound] government spending should expand up to the point at which full employment is restored. That’s not a guess or a statement of personal preferences, it’s a result.
I understand it, but how he arrives at it is above my pay grade.