Modern central banking is an empirical system--they do what seems to work. If inflation rises, then they must have been creating too much money. If deflation threatens, then they must not have been creating enough.

The billions that they're creating now are targeted at keeping short-term interest rates stable. When inter-bank rates (such as the Fed funds rate) move above the target, the ECB, the Fed, and other central banks inject enough money to bring the rate back down to the target. If interest rates fall below the target, they'll drain those reserves back out again to keep rates stable.

So, the reason they don't do it all the time is that they don't want to force rates below their target (which is their current best guess as to what rate will keep inflation low but stable).

The actual mechanism of the "pumping" is to lend money to banks against collateral. The collateral used to be either treasury securities, or else loans the bank had made. Over the past few days, though, the collateral has been mortgage-backed securities. I'd be really interested to know how careful the Fed is being to ensure that these securities are not the exact same (potentially worthless) ones that triggered the whole situation.