Many installs tell you to disconnect the negative battery terminal first. Why? Does this reset all the computer stuff?

 

yes, some of the stuff you put in, the computer needs to relearn the new modification to gain the max performance. it also is a safety issue, you never want to work on a car with the batter plugged in.

 

It does reset the computer to a degree but you don't loose your custom tunes if you have any. That is flash memory so it is flashed right into the computer and does not go away unless it is deleted or overwritten.