NEW YORK (CNN/Money) - The Federal Reserve has finally admitted that it's won the old war. How quickly can it gird itself for the new one?

Despite leaving rates on hold Tuesday, just like everyone expected them to, Fed policy makers set off a flurry of excitement among traders and economists in the statement they released after the meeting. The key bit was where they said that "the probability of an unwelcome substantial fall in inflation, though minor, exceeds that of a pickup in inflation from its already low level."

For a bank that has traditionally had its knickers in a twist over the possibility of higher inflation, this was a big shift. Ever since Paul Volcker stepped up to the plate in the waning moments of the Carter administration, the Fed has fought against inflation tooth and nail, bringing it down from double digits to the present low rates. Lower and lower inflation was the rallying cry, and Wall Street loved it: Such disinflation, as it is called, is the sweet spot of investing -- an environment where both stocks and bonds surge.

But eventually inflation can go too low, and the possibility of deflation, where prices fall, raises its head. What's so bad about deflation? It discourages households and businesses from spending. If you think that new washing machine is going to cost substantially less a year from now, you may hold off on buying it. This hurts the economy (the washing machine maker makes less, maybe it lays some people off), which ultimately can bring prices even lower.

In an effort to prevent deflation, the Fed appears to changing its doctrine from fostering disinflation to putting inflation in some comfortable range. The idea of such inflation targetting has been gathering steam lately. In an essay in the Financial Times two weeks ago, Goldman Sachs chief U.S. economist Bill Dudley and PIMCO managing Director Paul McCulley said the Fed should end its doctrine of disinflation and commit to keeping the fed funds rate low until the core Personal Consumption Expenditures deflator (Alan Greenspan's favorite measure of inflation) got up above 2 percent from the current 1.5 percent.

This, Dudley and McCulley argue, would take away expectations in both the market and the business world that the Fed is anxious to raise rates on the slightest sign of a pickup. As a result, short term rates would stay low, but -- since the possibility of a return of inflation would start seeming real, longer rates (like the 10-year Treasury yield) would pick up. Meantime, households and businesses, worrying that washing machines would cost them more later than they do now, would go out and spend.

Seems like the Fed was listening -- or more likely was thinking the same sorts of things. For investors, that's good news. It suggests that the Fed realizes it's fighting an unconventional war, and is ready to use unconventional methods.  

-- Justin Lahart is a senior writer at CNN/Money covering markets and investing.