The alternator rotor shim attaches the rotor to the crank. After every revolution, the rotor gives off one inductive ignition pulse, thereby determining the ignition timing. The Top Performances Lima shim is specially ground and allows the rotor to be offset towards the advanced position. The slightly earlier spark ensures more optimal timing and more performance-oriented combustion at higher speeds. This small, but significant component means you get a little more performance over the standard timing. This shim is also a good choice for de-restricted engines. Great idea for not much money. ATTENTION: do NOT install the wrong way around, otherwise the opposite happens.