The Federal Reserve's effect on the economy is visible when the interests rate target are maintained and the money flow is adjusted as needed to keep the rate at the desired level.
Varying the supply of reserves and changing the Federal Funds rate, the Fed can raise or lower interest rates in the economy. If it wants to stimulate the economy, it lowers interest rates to make loans cheaper. This encourages financial business investments and consumption. If it wants to slow the economy, raises interest rates.
This is explained in an Cbc article, written by Mark Thoma, who is a macroeconomist and econometrician at the University of Oregon. He specialises on finding out how monetary policy affects the economy. He also blogs daily at Economist's View.