when a swinging boom driven by a hydraulic cylinder stops , inertia causes continued motion of the boom which increases pressure in a chamber of the hydraulic cylinder . eventually that pressure reaches a level which causes the boom to reverse direction . then pressure in an opposite cylinder chamber increases until reaching a level that causes the boom movement to reverse again . this oscillation continues until the motion is dampened by other forces acting on the boom . as a result , an operator has difficulty in properly positioning the boom . to reduce this oscillating effect , a sensor detects when the cylinder chamber pressure increases above a given magnitude and then a determination is made when the rate of change of that pressure is less than a defined threshold . upon that occurrence , a control value is opened to relieve the pressure in that cylinder chamber .