From what I understand, a negative rep is where you have a LOT of weight on the bar (more than you can lift, if we are talking about bench pressing as an example). A spotter helps you lift it to the top position and then you slowly lower it to the bottom position. It has to be more weight than you can lift so you are actually fighting gravity instead of simply doing a controlled down motion.
Is this an apt description?
Sounds like it might be great to use with the incline bench press to help build up lagging upper chest muscles.