QE is the Federal Reserve buying long dated securities including MBS. The Fed pays the banks for these (and other long maturity securities such as Mortgage-Backed Securities ((MBS)) and Treasuries) and the payment is recorded as an accounting entry, by way of increasing the deposits banks hold with the Fed. The Fed pays the banks (sellers) a daily floating interest on this. This effectively means that by buying longer maturity Treasuries/MBS the Fed is reducing its long-term debt. Paying banks a daily rate of interest, which floats, is economic equivalent to issuing short term floating rate debt. So, if the Fed were a homeowner, this translates to the equivalent of a homeowner trading long term fixed rate mortgage (at very low rates) for (changing rates) short term floating rate mortgage. In the case of the Fed, it was around $ 40 billion/month.
Why is this not sensible?
First it does not make sense to replace very long-term low interest date with uncertain interest, daily priced, short term debt.
Second, it is not sensible for the Fed to buy mortgages when the housing market is booming. Few people own financial assets, especially MBS. So, purchasing financial assets helps just a few people for by buying MBS the Fed is propping up value.
Third, borrowing short to buy long term financial assets is both misguided and unsustainable in the long ter. The Fed should recognize inflation, problematic aspects of QE.