A valve actuation system for providing added motion in the valve mechanism of an internal combustion engine with at least one inlet valve and at least one exhaust valve is provided. The valve mechanism includes a rotatable camshaft including rising and falling ramps designed to interact with a valve control plunger inside a cylinder for actuation of the inlet or exhaust valve under the action of a valve spring. The cylinder communicates with a hydraulic fluid feed line via a check valve and with a hydraulic control circuit allowing a return movement of a valve to be delayed in relation to the timing of the corresponding falling ramp. The hydraulic control circuit includes a first and a second hydraulic fluid flow restrictor connected to the control plunger cylinder at different levels along its longitudinal axis. A selector valve enables a connection between the cylinder and hydraulic fluid return line to be opened via either one only or both flow restrictors.