Harmonic vibration dampener.
It's attached to the crankshaft pulley (bottom centre of engine). It's (very) basically a piece of rubber inserted into a pulley that stops resonance from destroying your engine.
What did that mean?
Have you ever seen someone dip their finger in water and rub it on the rim of a crystal glass? Hear that ringing sound; it's resonance. All objects have a resonance frequency, or a point at which they start to vibrate. Those waves are sometimes audible, sometimes not. In the case of your engine, the natural resonance frequecy occurs around 4500 rpm. At that point the crankshaft and block start to vibrate (not audible) with one another. That vibration can, over time, destroy the engine.
Underdrive pulleys (UDP) are marketed as "lighter". They are lightened by removing the rubber damper that comes on the stock pulley. By using a lighter pulley the engine is able to spin up faster because there is less weight (and thus friction) on the crankshaft. However, when you remove the damper you also remove the factory's method of stopping those harmful resonance waves from destroying the engine.
Now, the real question is, is the Ford damper really effective? Quite a few engineers have shown why those stock dampers are useful, and why not to use a UDP. There is evidence on both sides of the arguement that a UDP does no harm, and evidence that it does.
Chris has a UDP on his 95SE, and it has not caused any harm to the car so far. However, many people have had catastrophic engine failure that can be directly attributed to these pulleys. You have to make your own choice if you want one or not.
No one can honestly tell you that a non-dampened engine is better than one that is properly dampened. Unless of course they don't believe in physics. A better arguement for them might be that the stock damper on our cars simply does not cancel out all of the vibrations.
The dual-mode damper is an improvement, a factory supplied improvement, over the original Duratec crankshaft pulley. They supposedly make the engine run smoother and according to Terry Haines (the designer of our engine) work much better.
Given I have high mileage on my car (121,000 Km) I would invest in the damper for the sake of peace of mind. There are many more likely causes of engine failure than harmonic destruction but it will make me feel better to have the pulley installed.
I haven't checked up on the performance improvements this dual-mode damper offers yet. I'm not really interested either as I have a long commute and my engine spends most of it's time right at or just under 4,000 rpm. It's worth the investment to me.
