Six-and-a-half minutes before entering a Christchurch mosque and indiscriminately opening fire on the people inside, the man accused of the deadliest terror attack in New Zealand's history placed a camera on his head and began filming.

"Alright, let's get this party started," he told viewers, who were watching the camera's feed live on Facebook: "Remember lads, subscribe to PewDiePie."

Shortly afterwards, at least 49 people died in an assault on two mosques in the city. More than 20 others were taken to hospital with serious injuries.

Very few other words were spoken by the gunman during the 17-minute Facebook Live video, adding extra significance to the few words he did speak.

So why did he tell people to subscribe to PewDiePie, a vlogger and YouTube star with millions of followers?

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The phrase first began gaining traction online in October last year, when PewDiePie – whose real name is Felix Kjellberg – looked set to lose his title as the world's most popular channel to the Indian music label T-Series.

The rallying cry of PewDiePie supporters soon turned into a meme that saw people go to increasingly extreme lengths to keep PewDiePie the most-subscribed-to channel on YouTube.

From hacking printers, to vandalising a New York war memorial with the message, the message spread from online forums into the real world.

PewDiePie fans in Bangladesh placed posters of the popular YouTube star around their local neighbourhood ( Rokonujjaman Konok/Facebook )

The gunman's mention of the meme appears to be a nod to those in-the-know about internet culture.

A link to the Facebook Live video was first posted to the controversial online messaging board 8chan, where several links to his manifesto were also posted.

It appears the gunman recognised the best way for his extremist rhetoric to spread across the internet and reach as many people as possible was not only by sharing it through a variety of platforms – Facebook, Twitter, YouTube, and various file-sharing sites – but also through memes.

"I have provided links to my writings below, please do your part by spreading my message, making memes and shitposting as you usually do," someone believed to be the gunman wrote on 8chan.

These same internet companies that have allowed his video and manifesto to spread have rushed to take them down but it continues to pop up and be shared.

PewDiePie condemned the attacks and said he was "absolutely sickened" by the mention of his name in the video.

It shows that the "Subscribe to PewDiePie" movement is now way beyond his control.