Angina is a "spasm" of the artery, quite similar to toe cramps but are more serious of course, the nitro will act to relax the spasm by dilating the artery and promote a more steady blood flow {this is why the headaches}. But with Unstable Angina the nitro will sometimes be unable to relax it and the lack of blood flow will damage the heart muscle if left untreated too long. The "unstable" part comes from the fact that almost anything can trigger the spasm again, not just exertion, stress is a big factor but it's harder to "predict" which triggers to avoid when dealing with Unstable Angina.....time to chill sunny, at least for a bit