So, I was thinking of this line last night, and pondering the mechanics of it. I think we can safely assume that Kirk means here that what we think of as engine nacelles, atop their straight pylons, can be discarded in an emergency. Since this is well before Warp Cores are a thing, the Warp Drive in all it's explosiveness is assumed to live there, and presumably is meant to be a big bulky thing (like transistors!). Kirk thinks that ditching the nacelles, a last ditch maneuver, might reduce the ships mass and help them break out of orbit. My question is simple, then: if we were to have seen Scotty execute that maneuver on screen, what would it have looked like? Would the nacelles have been ditched, leaving the pylons they rest on bare? Or would the pylons themselves go with the engines? Personally, I think for the sake of ascetics you would have to separate the pylons along with the engines. A starship without nacelles is going to look weird either way, but less so without two now-useless pylons jutting out. I can also see where having the pylons contain separation engines to get the nacelles away from the ship would be a good idea, since they are meant in this era to be extra explodey (As Jeffries said, he didn't know what the engines did other than they were very powerful, and he reckoned that meant dangerous and possibly not a good idea to have within or near habitable sections of the ship). I'd be interested to hear what other people think, though!