Resurrecting this old thread.
I recently bought a dragster from a guy that has one of these Stop-it boxes on it. I became interested in what it did since the previous owner of my car swore by it and I found this thread.
I took the Stop-it off of the car and took it apart. It is indeed a diode and a resistor. The diode is a 1N5402 and the resistor is 3 watt 1 ohm 5% wire wound type. So I think the purpose is solely for the protection of other devices on the 12V circuit as Bill Harris has so accurately described. Like Bill says, any computer equipment built for automotive use will typically have this same protection built in to the device already. This makes the Stop-it device kind of redundant. Its interesting that Computech was recommending this type of circuit and they make computer equipment for automotive use, hmm.
I have read on another forum that the manufacturer claimed it will fix the "where did that come from" reaction times that racers sometimes see by limiting the rising amount of current that flows through a solenoid the longer it stays energized. I think that is baloney. The Stop-it device is to cure problems that could result after the trans-brake releases and won't affect the release of the solenoid at all.
http://www.v8buick.com/archive/index.php/t-198448.html
Once I scraped all the silicone off of the components in my box, I noticed that the resistor had turned brown, a sure sign of failure. I measured the resistance and sure enough, it was in the Meg-Ohms range, not the 1 Ohm range. So my box is actually burned up and probably had no affect at all even though the previous owner thought it did (kinda funny).
I noticed that the ShoGun web site is gone and perhaps out of business so I am unable to find the actual claims by the manufacturer. I did find a Stop-it that sold on ebay recently for $30 though.