I believe that the enforcement mechanism is that if they decide that they want to inspect you to make sure you have your tax-free goods with you, you won't get past exit Immigration until you do so. If you don't have your goods with you, they'd collect the tax before you'd be allowed to leave the country. So, how to collect it later isn't an issue. AFAIK, all the (voluntary) scan at the Customs counter (that most people skip because they don't even know about it) does is check to see if you've already been flagged in the computer for an exit Customs inspection, so that it can take place before you wait in the Immigration line. If you don't scan in advance, then the Immigration inspector will send you back to the Customs counter if they otherwise want to inspect you.

As to what flags you for an exit Customs inspection, who knows? I'd imagine it's some combination of: purchasing a large value of tax-free goods (such as the proverbial $100k watch), travel patterns (such as going back and forth to China every week and always having tax-free goods on your record), and/or a previous negative interaction with Customs.