The fact that it's a "dual" upgrade and there's a negative one. I believe it means that the upgrades will need to be used in pairs. So to assign the +1 to one of your ships you must assign the -1 to a different ship. Effectively you're moving around PS to where you want it.
Edit: Tried to type this out as quick as possible, but was ninja'd anyway lol
Edited by Zabuzauz