Hop ups, are things that make it better.....like, for instance, pkh, for your Rappy 50, you can got heliproz and buy a all metal head, some better servos, more metal stuff, carbon blades, and stuff like that...on a truck (R/c that is) you could put some metal guards on the bottom...

I think that you were asking in your post what hop-ups are.....but I have not seen any for the Nexus, I have seen some for the Rappy though.....PLENTY of those....