Martin,
I think you get the idea, but I'll add this little nugget that can snag you that is kind of unique to the JFK ER reserve. When you are up there sitting reserve, you may think that you are free and clear once the last departure goes out.(probably Shannon or something). Well any of those birds could do an air return for whatever reason and the crew could run out of duty, forcing them to call you out. That's probably no surprise...but...don't forget about the crapload of ERs coming out of ATL flying right over NY enroute to Europe. If any of them have to put down in JFK (MX, medical situation, etc.) they will be virtually guaranteed not to have enough duty to press on after their problem is fixed, so the short calls get the jingle at what could be a very late hour when you thought you were good to go, and cracked open a cold one about 3 hours ago, or jumped on a flight home. Back when I was on resreve, on occasion, I could get the scheduler to release me for both of the reasons I mentioned, but it was hit and miss. Miss most of the time.
Granted, fairly rare but could really catch you off guard.