It's bull****. If you can provide a link, I can tell you with certainty.
Basically they are little resistors that when spliced into a specific wire for fuel control, lean the car out a little, giving you more power. I also heard most ECU's will tune them out, or the resistor will fail.
most are resistors that replace your IAT sensor - raising the resistance and 'tricking' your ECU to thinking it's colder outside and so the car tends to over fuel. This does not work on modern cars that inputs from many sensors, and evenutally cause a CEL error thinking the snesor is dead.
I bought something like this off eBay many years ago just to see what it was. They sent me a resistor tapped to a instruction sheet that didn't even work for my vehicle (despite it being mentioned in the ad). The horsepower increase they advertise varies based on the number they pull out of their rear on a given day.