FireChat, an app that lets users chat without a cellular or internet connection, has been downloaded more than 100,000 times in Hong Kong amid mass pro-democracy protests. Here's what you need to know about it.

What is FireChat?

FireChat is an iOS and Android app that lets smartphone and tablet users chat even without a cellular or internet connection. That's why it bills itself as "off-the-grid."

However, it can also be used to chat over the internet, via its "everyone" mode.

The app was developed by a San Francisco-based company called Open Garden.

How does it work without an internet connection?

A screenshot of the FireChat app is seen from the Apple App Store. Open Garden says FireChat is ideal for situations where Wi-Fi and mobile phone coverage aren't available, such as on the subway or in the wilderness. (Open Garden)

In "nearby" mode, the app uses Bluetooth to connect to nearby phones that also have the app installed. If lots of people have the app, they form a "distributed" or "decentralized" network. A message can be passed from phone to phone in a daisy-chain-like fashion, connecting users who are farther away from one another. The company describes it as "crowdsourcing the connectivity of those around you."

When might you use a service like Firechat?

According to the Open Garden, FireChat is ideal for situations where Wi-Fi and mobile phone coverage isn't available, such as on the subway or in the wilderness, or in crowded places such as concerts or protests where cell towers may be swamped.

The device also allows you to connect anonymously to strangers around via "firechats" – live group discussions of up to 10,000 people.

How close do users need to be to one another?

When the iOS app was first released in March, Open Garden wrote on its blog that the nearby mode works best within nine metres of your location. The description for the Android version of the app in the Google Play store says nearby mode can be used up to 60 metres away. However, users can connect over longer distances via other users.

In 'nearby' mode, the app uses Bluetooth to connect to nearby phones that also have the app installed. If lots of people have the app, they form a 'distributed' or 'decentralized' network that allows messages to be passed from phone to phone over longer distances. (FireChat)

How popular has the app been among Hong Kong protesters?

The company's co-founder and CEO told the South China Morning Post that 100,000 people in Hong Kong had downloaded the app in 24 hours from Sept. 28 to Sept. 29.

"What we're seeing right now is beyond anything we've seen," Christophe Daligault, the marketing officer for Open Garden, told Reuters on Sunday.

According to the Post, protest organizers encouraged demonstrators to download the app after hearing rumours that the Hong Kong government would shut down cellphone networks in the district where the protest was taking place.

Reuters reported that Hong Kong's wireless networks were disrupted Sunday night, possibly from being overloaded.

The app had previously also been used to some extent during internet blackouts in Iran and Iraq earlier this year.

What is the app being used for?

Reuters says FireChat was used as an alternative to send updates about the protest and the latest police movements.

However, critics such as Masashi Crete-Nishihata, research manager at the University of Toronto-based Citizen Lab who goes by the Twitter handle @grayops, note that its functionality is limited.

The Citizen Lab, based at the university's Munk School of Global Affairs, conducts research on information technology, human rights and global security.

One review on the Google Play store notes that FireChat is "more a chat room" than an instant messaging platform.

Is it secure?

No. According to a Citizen Lab report posted in July, it's "highly insecure for sensitive communications." The information is not encrypted, and is stored on a person's device, allowing anyone to easily display recent messages. The app also makes it "relatively easy to impersonate another user without authorization," CitizenLab says.

Open Garden has warned Hong Kong protesters not to use their real names.