Such measures work mainly through a portfolio balance effect, increasing demand for these securities above what it would otherwise have been, thereby lowering their yields. There are, of course, also flow-on effects to other asset prices. Purchases of long-term securities can be undertaken in association with, or independently of, quantitative easing.
In the former case, the central bank would buy long-term bonds and pay for them by adding to banks’ reserves. In the latter, it would buy long-term bonds but pay for them by selling some of its existing holdings of short-term securities, or by issuing its own securities, thereby leaving reserve balances unchanged.