As long as interest rates are above 0% QE pumps money into the system. If you bought a 10% yield 10-year bond last year and that yield went to 0% it means you

As long as interest rates are above 0% QE pumps money into the system. If you bought a 10% yield 10-year bond last year and that yield went to 0% it means you can sell that bond today for the same price you'd get in maturity. So that's a pretty good deal. The government or the central bankers are basically giving free money to bond holders on the assumption they will go use that money to stimulate the real economy. I think the last few years prove that it works to some degree but definitely a lot of that money gets stuck in various places, e.g. the banks. During QE the central bank can also acquire other assets that are riskier than bonds and take risk off other people's balance sheet, again hoping that money will be used to stimulate the real economy. At 0% it pretty much stops working. Up to 0% some bond holders are still holding on because they are betting on that being the better investment. The other problem is how do you exit QE. We're going to find out how that works over the next few years...