Originally Posted by Z1Performance

They are essentially strut braces with a small shock inside. They mount in the front bumper, at the peak of the frame rails, and then in the rear spare tire well. Their job is to dampen vibrations and harmonics that are a natural byproduct of running stiffer springs and more aggressively valved shocks (and lower profile tires) as the NISMO Z comes with. I have the front one from the NISMO 350Z on my car (the rear wont fit due to my sub box). It is a completely dramatic change (for the better) vs not running it. My car was never uncomfortable, despite its very low stance and low profile tires (HKS coilovers and 265/30, 185/30 RE050A), but with the front damper installed, the car was a pleasure to drive on our crappy local roads. So much so that it is more composed than my daily driver ('07 9-3 Aero Wagon).

Here is a review from one of our Subaru STi customers who had them on his car (Yamaha makes them for Subaru, Nissan and I believe there are a couple Toyotas that have them too)

Z-1 Performance Automotive Services Inc. - STI Perfomance Strut Dampers