I cannot reveal the (technical) details.

For app developers, promocodes are a wonderful thing for 1) customer service, 2) media outreach, 3) reaching out to app reviewers. Its painful when you switch from selling standalone app to In-App-Purchase, because Apple doesn't have promocodes. But we couldn't do without it, so we built it into the app.

Is it against Apple's Terms&Conditions? I don't know. Regardless, its #HackerPride!