An explanation over at Understanding the Market:

Obama’s economic team use the payroll data (see here for their report). As of the end of December, the BLS estimated that businesses employed 135,489,000 workers (what the Obama advisers call “payroll employment”). They estimate that with the Obama stimulus plan, employment will rise to 137,550,000, resulting in an increase of roughly 2 million more jobs; but without the stimulus, employment would only be 133,876,000, so the Obama plan results in almost 4 million more jobs than would otherwise be the case.
The Obama team presents these numbers as the result of forecasts of what will happen without the stimulus compared to what will happen with the stimulus. Here are the job data for the last five recessions and the estimates for the current recession.

There’s a lot more Manzi-bait over there.