How about if you have proper odds to call you are drawing to the nuts and if you don't you are chasing.
^^^ this pretty much. It's not about pot odds or anything or the sort. Think of "on a draw" as a noun and "chasing" as a verb. What you have (noun) is a draw and what you are doing (verb) is chasing the card you need to make your draw.No difrence excepted the way its used.
Im drawing, or im on a draw vs That dumb ****ing donk was chasing.......