as stated in the website, it's just a capacitor. the idea must have come from the capacitor installed across the pos and neg terminals of a ac/dc transformer to convert the wave output from fluctuating pos-zero sinusoidal wave (such as that produced by a vehicle alternator) to a flat waveform (like that produced by a battery).

BUT the question is, should this not be installed across the alternator terminals and not across the battery terminals? you will still have the spikes across the alternator terminals even with this gadget installed as in the picture, so then the whole vehicle electrical system will still have those spikes. the output from the battery is already constant and flat so why put it there? wrong placement of a good idea imho.