The Fed would inject or drain reserves through open market operations to keep the vertical supply curve at the desired location—where it intersects the demand curve at the intended federal funds rate. Since the supply of reserves naturally changes—for example, because of the withdrawal of reserves into physical currency—the Fed conducted these operations frequently.
Today, being in an ample-reserves regime, the Fed operates on a different segment of the demand curve and uses different tools, as illustrated in the right panel of figure 4.