Method for operating an actuator via a pressurized fluid circuit to move a valve pin of an injection nozzle for molten material within a mold, wherein the valve pin moves from a closing position, where no molten material passes through the nozzle, to an opening position, where molten material passes through the nozzle, and vice versa, wherein the actuator comprises a chamber and a piston that is movably mounted in the chamber , linearly displaced by the thrust of the fluid and connected to the valve pin to move it, wherein at least one auxiliary tank (60; 560), comprising a first and second fluid chamber (66a, 66b; 566a, 566b) separated by a movable wall (64; 564), is connected to the chamber of the actuator , the first chamber (66a; 566b) of the auxiliary tank (60; 650) being fluidically connected to the chamber of the actuator , so that the displacement of the movable wall results in a predetermined amount of fluid being sent from the first chamber (66a; 566b) of the auxiliary tank (60; 560) to the chamber of the actuator , or vice versa with a reverse displacement of the movable wall, and the second chamber (66b; 566a) of the auxiliary tank (60; 560) is connected to a pressurized fluid source to move the movable wall and bring the valve pin to the closing position, with the steps of - detecting an event which is external to the first and second chamber (66a, 66b; 566a, 566b) of the auxiliary tank (60; 560) and indicative of the condition that the valve pin is traveling toward the closing position, and - making said first and second chamber communicable with each other only if and when the event is detected. wherein the valve pin moves from a closing position, where no molten material passes through the nozzle, to an opening position, where molten material passes through the nozzle, and vice versa, wherein the actuator comprises a chamber and a piston that is movably mounted in the chamber , linearly displaced by the thrust of the fluid and connected to the valve pin to move it, a chamber and a piston that is movably mounted in the chamber , linearly displaced by the thrust of the fluid and connected to the valve pin to move it, wherein at least one auxiliary tank (60; 560), comprising a first and second fluid chamber (66a, 66b; 566a, 566b) separated by a movable wall (64; 564), is connected to the chamber of the actuator , the first chamber (66a; 566b) of the auxiliary tank (60; 650) being fluidically connected to the chamber of the actuator , so that the displacement of the movable wall results in a predetermined amount of fluid being sent from the first chamber (66a; 566b) of the auxiliary tank (60; 560) to the chamber of the actuator , or vice versa with a reverse displacement of the movable wall, and the second chamber (66b; 566a) of the auxiliary tank (60; 560) is connected to a pressurized fluid source to move the movable wall and bring the valve pin to the closing position, with the steps of - detecting an event which is external to the first and second chamber (66a, 66b; 566a, 566b) of the auxiliary tank (60; 560) and indicative of the condition that the valve pin is traveling toward the closing position, and - making said first and second chamber communicable with each other only if and when the event is detected. - detecting an event which is external to the first and second chamber (66a, 66b; 566a, 566b) of the auxiliary tank (60; 560) and indicative of the condition that the valve pin is traveling toward the closing position, and - making said first and second chamber communicable with each other only if and when the event is detected. <EOS>