The vertical speed indicator displays the rate of change of altitude to the pilot in feet per minute. In the atmosphere density and static pressure decrease as altitude increases, for demonstration purposes let's assume that at low level altitude static pressure has a value of 3 units the middle level has two units and high level is only one unit, the VSI consists of a flexible diaphragm which expands or shrinks like a balloon when the static pressure changes, the static pressure is fed to the diaphragm by the static port and a mechanical link drives the pointer to display the rate of climb or descent to the pilot. The diaphragm is housed inside a case which is connected to the static port via the calibrated leak, this connection is designed to slow down the pressure change inside the case, whereas the pressure change inside the diaphragm is almost instant.

For an aircraft flying at the middle level altitude static pressure from the static port is two units inside the case and two units inside the diaphragm, both pressure areas are equal thus cancel each other out, the diaphragm does not move and the VSI displays zero, when the aircraft descends through the low-level altitude at this point the pressure in the diaphragm increases immediately to three units, the diaphragm expands and the VSI indicates a rate of descent, the pressure inside the case still remains at around two units since its increase is delayed by the much smaller calibrated leak, when the aircraft levels off at low level altitude both pressure areas eventually equalize and the VSI returns to indicate zero, when the aircraft climbs through the high level altitude at this point the pressure in the diaphragm decreases immediately to one unit, the diaphragm shrinks and the VSI indicates a rate of climb, the pressure inside the case still remains at around two units since its decrease is delayed by the much smaller calibrated leak, when the aircraft levels off at high level altitude both pressure areas eventually equalize and the VSI returns to indicate zero, should the static port get blocked both pressure areas inside the VSI will remain equal and the pointer will indicate zero whether the aircraft descends or climbs.