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A Harmonic Balancer is used by some engines to dampen vibrations. It is used instead of the front crankshaft pulley and has a rubber-mounted heavy outside flange. The A-series does not need a balancer, as the engine is precision balanced at the factory, and its small bore produces relatively little vibration compared to big-bore engines.
Overview
If you are running over 7500 RPM, a balancer may be useful.
Custom A-series harmonic balancer
At racing RPMs the balancer is credited with eliminating the "flywheel coming loose" problem that A-series engines sometimes have.
Also see: Flywheel
A harmonic balancer is like a small flywheel at the front of the engine, except that it is mounted in rubber or other elastic material & is integrated with the front pulley. As the rotating crank accelerates & slows during a single rotation, or as Aiden correctly put it "loads and unloads in the twisting direction", this 'little flywheel' either lags behind the acceleration, or over runs the deceleration of the twisting/rotating crank & it can do so because of its flexible mounting ring. This has the effect of damping out the worst of the sharp changes of rotational velocity & as the name suggests, it helps to balance out the harmonic frequencies in the crank & in so doing it can save the engine from a nasty incident at speed.
It pretty much eliminates flywheel bolts loosening, plus would reduce fatigue in other bottom end components. It cost $350 from Ross Balancers in blacktown, cheap insurance for a racer in view of some.
Also see this discussion: harmonic balancer