A method of operating a microsurgical instrument, such as a vitrectomy probe, is disclosed. The probe includes a port for receiving tissue and an inner cutting member. A flow of tissue is induced into the port with a vacuum source, and the member is actuated in a cyclic manner to open and close the port over a plurality of cut rates. A duty cycle of the member is varied with the cut rate to vary the flow of the tissue into the port. The method yields the ability to adjust flow rate into the probe and aperture of the probe for a given cut rate so as to effectively perform a wide variety of vitreoretinal surgical objectives.