As I understand it, you need to go to the correct place to get the local code (two separate people fwded a link to me, one worked, one didn't). That will require name, passport number, phone number, and maybe some other critical info (like your favorite flavor of ramen and how much baijiu is required to put you under the table). Contact the local entry/exit bureau - they should be able to provide the correct link or else hook you up with someone who can.
You'll get a text message - you have to enter the code. If all goes well you will then have a magic green QR code.
On the other end of any planned trip, you may end up exiting normally, or you may have to scan another QR code and get another text message with a code you need to enter. If that works, you get a magic green arrow. At that point, you may or may not get pulled out of the line for more info to be taken.
Oh, almost forgot. Local PSB dropped by my hotel and took photos of my green QR and green arrow. No idea how common this is.