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.