Twitter’s acquisition of India-based Zipdial has got a lot of people scratching their heads. What, exactly, the company does is a mystery to many.

While it may describe itself as a “marketing and engagement service,” what Zipdial does best is known as missed-call marketing. An obscure mobile offshoot of permission marketing, missed-call marketing is just what it sounds like: Consumers opt into ads by dialing a number and hanging up before the call connects. The customer will then receive text messages and calls with ads in them. Which, naturally, leaves us with plenty of other questions, starting with:

Why?

India was unique in the way that it progressed when it came to mobile technology. For most people, unlimited calling plans are too expensive. Most people instead use prepaid cellphone plans, which strictly limit calling minutes but not incoming calls. So if you want to save your minutes, you’d make a missed call. One CPRSouth survey found that 65 percent of Indian mobile phone users prefer a missed call to an actual phone call. (People also use it for non-advertising purposes, like letting a friend know you’ve arrived to pick them up.)

So missed-call marketing is a rooted cultural thing?

It’s essentially a smart hack by marketers looking to speak to a population that is connected but with limits.