This policy is usually invoked when the normal methods to control the money supply have failed, i.e the bank interest rate, discount rate and/or interbank interest rate are either at, or close to, zero.
A central bank implements quantitative easing by first crediting its own account with money it creates ex nihilo ("out of nothing").
It then purchases financial assets, including government bonds, agency debt, mortgage-backed securities and corporate bonds, from banks and other financial institutions in a process referred to as open market operations.
The purchases, by way of account deposits, give banks the excess reserves required for them to create new money, and thus hopefully induce a stimulation of the economy, by the process of deposit multiplication from increased lending in the fractional reserve banking system.
Risks include the policy being more effective than intended, spurring hyperinflation, or the risk of not being effective enough, if banks opt simply to sit on the additional cash in order to increase their capital reserves in a climate of increasing defaults in their present loan portfolio.
