I don't know the logistics behind it. You put in the ADR you are looking for and it checks like hourly and if it finds something it will email or text you to let you know. I actually heard about it on these boards
I've used the ADR finder, my ADR 180 opened up 2 days ago, and already I was able to "improve" my reservation by booking a time that is closer to my preferred time
You can have up to 10 searches going at one time, you put in the restaurant and the time, and it searches within 1 hour of your preferred time, and will SMS/email you when it finds it.