Abu Yusuf al-Britani
17-year-old Talha Asmal - also known as Talha Asmal - is believed to have become Britain's youngest suicide bomber in IraqTwitter/Anna Ahronheim

In the last photo taken before he blew himself up in a dusty battlefield in northern Iraq, 17-year-old Briton Talha Asmal sat in the front seat of a car with a single finger on his right hand pointing skywards.

The salute has become common in Islamic State (Isis) propaganda, both from the battlefield and in the final minutes before a shaheed – or martyr – embarks on a suicide bombing. But what does now ubiquitous hand signal mean?

The single raised index finger has so far escaped analysis, likely due to the fact that those making the gesture have often had a severed head in their other hand, but refers to the first half of the shahada, the affirmation of Muslim faith that his recited before every prayer.

Abu Ali al-Shishani ISIS Lebanon
Anas Sharkas, aka Abu Ali al-Shishani,making "the one" salute in a video from Lebanon.YouTube

It is the passage that so defines the Muslim faith: "There is no god but Allah, and Muhammad is his Prophet."

"The one" was actually a recognised gesture by Muslims long before IS and signified the tawhid, the belief in the oneness of God. It would traditionally be used during prayer or sermons.

"The shahada is the definitive Muslim doctrine. It is like the trinity for Christians. But it has been taken over by the Islamic State," said Veryan Khan, editorial director at TRAC Terrorism, which monitors IS propaganda.

The salute was used early on in the days of IS and even by leader Abu Bakr al-Baghdadi during his first speech at Friday prayers after the takeover of Mosul.

It also has a solid jihadi pedigree, said Rita Katz, director of SITE Intelligence Group.

"The gesture has been used by jihadis for years, including high profile ones like Osama bin Laden. Within the jihadi context, the raised index finger takes on political meaning as well, widely rejecting any form of government not under Shariah law," she said.

Since then, it is not only used by fighters and supporters of IS but by those celebrating weddings in IS-held areas.

Like the Nazi salute or the raised fist, the gesture has become an integral part of IS propaganda, particularly among the young. Videos from IS-held areas frequently show small children saluting with a single finger in unison while shouting IS propaganda.

"It so much reminds of the sieg heil [Nazi salute] when you see the little kids do it simultaneously," said Khan.

Abu Bakr al-Baghdadi
Abu Bakr al-Baghdadi speaking at Friday prayers during Ramadan at a mosque in Mosul, IraqYouTube

A unity of gestures and symbols has become increasingly important to IS, with its black and white flag used by IS-affiliates across the Middle East, a factor that helps the terrorist group distinguish itself from the dozens of competing jihadi factions across the region.

It also helps bolster the central idea of the caliphate, that it is a unified Islamic state that stretches not just across swathes of Iraq and Syria but the wider world.