NPR's Planet Money program has a pretty good piece headlined "How To Spend $1.25 Trilllion." From the article:
In the end, they came very, very close to their target: They told us they were just 61 cents short. (In other words, they bought $1,249,999,999,999.39 worth of mortgage-backed bonds.)
The Fed was able to spend so much money so quickly because it has a unique power: It can create money out of thin air, whenever it decides to do so. So, Dzina explains, the mortgage team would decide to buy a bond, they'd push a button on the computer — "and voila, money is created."
The radio piece, which is worth listening to if you have the time and are interested in the Federal Reserve, refers to it as "magical."