As others have said it sounds like 12v capacitor filtering system , and seems small if it is .
The thing the ECU have self regulated voltage for critical sensors (5v ref voltage in most systems) and the battery does good job of filtering the 12v . Only way I can believe these types of filters work if either battery was very small or bad condition for vehicles load or vehicle wiring had bad grounds or something preventing properly working charging/12v systems .
If you look at this PDF in one of links provided above .
They show how pulse width of injector is smaller with the unit installed. Ok I not going to dispute that but the O2 sensor is in closed loop and IMO would of kept A/F the same (within some % of error)during cruising .
http://www.fueldoctorusa.com/pdf/BOS..._3_results.pdf