as the entire part which mounts to cam the phaser is typically made up of rotor and housing and possibly spool valve and check valves a piston phaser is a phaser actuated by pistons in cylinders of an internal combustion engine rotor is the inner part of the phaser which is attached to a cam shaft pulse width modulation pwm provides a varying force or pressure by changing the timing of on off pulses of current or fluid pressure solenoid is an electrical actuator which uses electrical current flowing in coil to move a mechanical arm variable force solenoid vfs is a solenoid whose actuating force can be varied usually by pwm of supply current vfs is opposed to an on off all or nothing solenoid sprocket is a member used with chains such as engine timing chains timing is defined as the relationship between the time a piston reaches a defined position usually top dead center tdc and the time something else happens for example in vct or vvt systems timing usually relates to when a valve opens or closes ignition timing relates to when the spark plug fires torsion assist ta or torque assisted phaser is a variation.