Know as quantitative easing (QE), it involved almost $US4 trillion in financial asset purchases – mainly US government Treasury bonds - from banks and other holders of the securities.

The Fed also bought asset-backed securities such as repackaged loans from banks and other issuers.

The October Fed open market committee meeting is expected to rise on Thursday morning with a statement by chairwoman Janet Yellen confirming the end of QE in the US, and offering some indication of when the Fed might start lifting interest rates.

Bets on when the Fed will start tightening monetary policy, after six years of extreme easing, are scattered throughout the second half of next year.

Any pre-emptive move could choke off the US recovery and trigger wide-scale market volatility as investors take new positions, so the Fed's normal caution on language is likely to prevail in its statement on Thursday.

The aim of QE was to drive down long-term interest rates and also push cash into the real economy, encouraging business investment and consumer spending.

Central banks normally help stimulate economic activity or curb inflationary pressures by setting a target short-term rate, which they then support by buying and selling short-dated securities.

This both provides a reference short-term lending rate for banks and debt issuers and controls the quantity of money in the economy.

Monetary policy can also influence the price of assets such as shares or real estate by providing investors with cheap money that can be used to chase higher, though riskier, returns. At the same time, low interest rates often equate to a weaker currency, which can help exporters and import-exposed companies.

However, once short-term rates hit zero, or close to zero, there are few other conventional tools central banks can use to encourage economic activity. And this is where QE comes in.

The current asset-buying program, known as QE3, started in September 2012 with purchases of $US85 billion a month, gradually tapering off by $US10 billion a month this year to a scheduled final purchase this week of $US15 billion.

Some critics say the first program was effective, while the second two simply provided cheap money for professional investors to chase yield in a world of low interest rates and low growth.

It also kept US bond yields and the US dollar low, which hurt the export competitiveness of countries around the world, including Australia.

"QE is only effective when financial markets are dysfunctional," said Perpetual Investments' head of investment market research Matt Sherwood.

"That was the case in 2009; it was not the case in QE2 or QE3."

In any case, the Fed's own measure of QE's success has always been the health of the job market, which is currently in much better shape than in the aftermath of the GFC.

When QE3 started, unemployment was above 8 per cent; it now sits below 6 per cent.

Barring an extension of the scheme, this week's final bit of tapering could prove anti-climactic – which is exactly what the Fed wants.

The last time the US central bank was too explicit in describing how it would taper off QE, investors dumped emerging market assets en masse, driving down currencies and opening up current account deficits around the world.

Known as the "temper tantrum", it stemmed from investor panic at the thought of a rapid transition from near-zero per cent interest rates in the US to a series of rate increases that would quickly reassert the greenback as the reserve currency of choice.

The fright also drove up long-dated bond yields, which killed demand for mortgages, one of the mainstays of the US recovery.

So far this week, global markets have been relatively calm, not least because continued economic weakness in Europe and Japan will force their respective central banks to broaden their own versions of QE.

This, according to Sherwood will keep bond yields – and so long-term interest rate expectations – around the world low for the foreseeable future.

"In the end there's probably not going to be much of a reaction to the Fed taking its foot off the accelerator, given that we have the European Central Bank and the Bank of Japan putting their foot down on the accelerator," he said.

Nor should it disrupt equity markets, he said.

"The tapering should not have a marked impact on the global share market provided the US Fed maintains its guidance that US rates hikes are a late-2015 story."