If you've ever been crammed into a stadium alongside thousands of screaming football or music fans, you already know what the tens of thousands of demonstrators pouring into Hong Kong this week are learning: when you pack that many people into a tiny area, your phone's Internet grinds to a halt.
Smartphones should make it easier to organize protests, but they're as good as bricks when cell towers get overloaded with traffic or when governments decide to flip the switch. Hong Kong has seen both of these happen: Thousands of people on the street means mobile Internet is useless in packed areas, while Chinese authorities are blocking Instagram on the mainland, favored by Chinese dissidents because it was one of the few social networks not blocked in the country.
In the face of these hang-ups, Hong Kong's demonstrators have turned to FireChat, a smartphone app that allows users to communicate even when they can't get online or send texts. Unlike chat programs that work over the Internet, FireChat connects directly to other nearby users within up to about 250 ft. More people in range can then join the chat, extending the network even further. Pretty soon you can get up to a few thousand people chatting away, all without anybody connected to the Internet.
FireChat is based on mesh networking, in which every device on a network works as a node for expanding that network. The idea's been around for decades, now popular as a way to communicate during disasters like hurricanes. But Hong Kong shows it's useful during civil disobedience too. Some 200,000 people there downloaded the app between Sunday and Tuesday, says Micha Benoliel, CEO of Open Garden, the company behind FireChat, sending it skyrocketing to the top of the region's app-store charts.
Speaking from Hong Kong, Benoliel tells TIME that FireChat's sudden popularity there isn't a "complete surprise" because it was also popular with Taiwanese protesters last March. It's also the latest in a long line of technologies that helped fuel wide-scale protests. Iran's 2009 Green Revolution was dubbed the Twitter Revolution, thanks to protesters' penchant for organizing via Twitter, likewise 2011's Occupy Wall Street was a hashtag before it was a street protest. Facebook and YouTube, meanwhile, have brought us to the front lines of the Arab Spring and Syria's long-fought civil war, even being used as recruiting tools by antigovernment rebels and jihadist groups. Where Twitter, Facebook and YouTube all fall short, however, lies in their need for an Internet connection to work — not the case for FireChat.
Still, FireChat isn't perfect for protesters. The chat rooms are open, making it easy for a first-timer to join — but that first-timer could also be a local authority poking around at the goings-on. However, Benoliel says the company is working on protester-minded updates like private messaging and encryption, as Open Garden advocates for "freedom of speech and access to information."
"If this application can help in this way, it's very aligned with the mission of the company," Benoliel says. "[FireChat] hasn't been built for that purpose, but if it can help people in that situation, we are very supportive of what's happening here in Hong Kong."