I used it last time through Miami. It is awesome. The app is called mobile passport. It is marked on the signs now. I think it was a yellow symbol and said mobile passport control.
Basically it replaces the electronic kiosk where you line up with everyone, and answer the questions, it takes your picture, and prints a receipt. You do all of that on your phone and take your own picture. It generates a QR code that is scanned in a specific mobile passport line.
I bypassed about 300 people in line last time. You just show the app to the person putting people in line and they direct you through.
You go to the next person who normally looks at your receipt and tells you which line to go in and they point you to the mobile passport designated line.
When I got to the line, it was me and two flight attendants. No wait. No questions. They scanned my code and I walked through.
One of the officers did tell me that they have had problems with it not working consistently though, but I experienced no problem.
Honestly, it saved so me so much time that I wasn't going to tell anyone.