Two different events put a sold-out campsite back on the market, and they behave nothing alike. A scheduled release is inventory the park opens on a fixed calendar — Recreation.gov at 4:00 PM ET, ReserveCalifornia at 8:00 AM PT. A cancellation is somebody giving up a booking they already hold, and it can land at any hour.
Scheduled releases are predictable, which also means they are competitive: everyone knows the time. Recreation.gov opens a rolling six-month booking window, and ReserveCalifornia roughly six months as well, so the sites appearing at the drop are for dates half a year out. If you are booking far ahead, this is the event that matters and being logged in and ready at the exact minute is most of the game.
Cancellations are the opposite. They are unpredictable, far less contested, and they are the only realistic route to a sold-out weekend that is weeks rather than months away. They cluster loosely around cancellation-fee deadlines and in the last week before a trip, when plans fall apart — but there is no schedule to memorise, which is exactly why watching manually does not work.
Kestrel handles both. Every watched campground is rescanned every two minutes, and every 30 seconds for half an hour either side of the provider's release time. For a near-term trip, set the alert and stop refreshing. For a release-day booking, set the alert and also be at your keyboard.