I just watched The Enterprise Incident for the very first time. What I didn't get was what was so revolutionary about their cloaking device this time around that made the Federation act in order to get their hands on it? They had it when we first saw the Romulans in Balance of Terror, so what was new?
You may recall in "Balance of Terror," the
Enterprise was capable of tracking the cloaked Romulan ship using motion detectors. As long as the ship was in motion, it left some kind of telltale sign that allowed it to be tracked. (They matched the Romulan ship move for move like an "echo.") The cloaking device was an invisibility screen--Spock said "the selective bending of
light." You couldn't
see a cloaked ship, but there was all kinds of other stuff besides visible light that allowed its existence to be detected by sensors. It was when the Romulan commander jettisoned some material--and a bomb--that the
Enterprise lost track of the Romulan ship. (The Romulans held perfectly motionless so there was no way to figure out where they were. I guess it's sort of like holding perfectly still when the
T. Rex is around because it keys in on motion.) The point is the cloaking device doesn't allow you to
see the ship. But it doesn't keep your sensors from detecting heat or motion or magentism or gravity or maybe even subspace distortion--just visible light.
At any rate, Mister Spock doesn't say in "The Enterprise Incident" that the Romulans now have a cloaking device. That would be silly. Eveyone already knows they do. It seems the Romulans have now
improved their device and even the Romulan ship's motion doesn't give it away and allow it to be tracked the way it could be tracked in "Balance of Terror:"
"I believe the Romulans have developed a cloaking device
which renders our tracking sensors useless."
Or maybe to clarify a bit what Mister Spock meant:
"I believe the Romulans have developed a [better] cloaking device which renders our [motion] tracking sensors useless [when we had previously been able to use them to track a cloaked ship. So not only can we not see it, we can now no longer detect it other ways either like we used to be able to.]"