Most...lol
It’s, in a nutshell, an acknowledgement of the faith/one god and often seen under duress/stress or as a last act of faith before dying (like by getting your head sawed off by another who follows islam V 2.x)...
Around these parts, it means ‘pull my finger’
I give ISIS the same one finger salute I give to the democrats and henry barbour.
This hand gesture represents shahada, the first pillar of Islam - emphasizing that their is only one God. Strict monotheism (no saints, no undue elevation of individuals, gravesites, etc.) is a particular point of emphasis for the puritanical/fundamentalist Sunni sects. It is the main doctrinal reason they cite for killing Shia Muslims - committing the inexcusable sin of “shirk”, attributing Godlike characteristics to anything but Allah (e.g. performing miracles, answering prayers, granting wishes, adoration that is like worship, etc.).
This hand gesture is not unique to ISIS, but it is characteristic of them.
Desert Arabs communicate by gesture more than most folks. It is a cultural thing with them. If you tell one yes, but make the hand gesture for no, they tend to mostly treat it as no.
It also has a bit of history as a kind of wordless battle cry - firing up the holy warriors (jihadis)in their certainty of their righteousness, as they close for battle or execute a prisoner.
Shhhh, don’t upset the paranoid fools. We all know the index finger is going to start WW3!!!