It disables the spark advance to the distributor by removing the signal from the ecm to advance the spark. That's a neat idea of monitoring the voltage there but there is a lot more variables there that would affect the advance besides temperature sensor. What year vehicle does your friend have? Even if its an 80-90s truck/van, there are multiple spark tables involving temperature, rpm, load, and air/fuel to name a few. If its a newer vehicle, there are even more.