The Fed injects money into the economy by buying US Treasuries or GSE MBS traditionally from the open market (QE). During continued QE the Fed will re-purchase

The Fed injects money into the economy by buying US Treasuries or GSE MBS traditionally from the open market (QE). During continued QE the Fed will re-purchase these assets when their existing ones mature. During QT on maturity rather than buy assets again they write them off and liquidity (money) is removed from the market.