In principle, intervention can be initiated by either party, although when the Treasury opts to intervene it is the Federal Reserve Bank of New York that actually does the buying or selling of foreign currency, albeit from an account held in the name of Treasury and at the direction of Treasury.
Similarly, when the FOMC makes a decision to intervene, it directs the Federal Reserve Bank of New York to do so, from the account in the name of the Federal Reserve System. The traditional practice is that U.S. intervention exercises are carried out jointly, half from the Federal Reserve’s account and half from the Treasury’s account.