"Muslims began pointing one finger up in imitation and mockery to indicate God is singular and not a Holy Trinity. So one finger up means Islam." - Trumpinator
I would also add: The phrase 'allahu akbar' is Arabic for "allah is greatest." Truth and power in Islam are identical. It is not a religion of the oppressed, but of the oppressors.
Mohammed's prophecies are validated by his conquests. The truth of Islam is seen in the expansion of Islam. When Muhammedans succeed in killing non-muslims and those they accuse as apostates, they prove the truth of 'their' version of 'their' religion.
That is why Muhammedan terrorists shout, "allahu akbar", "allah is greatest." allahu akbar doesn't mean allah is great, in a "Isn't 'allah and the Virgins of Paradise' a great band". It's more like allah is greatest or superior to all others.
The message of allahu akbar is the message of the qu'ran 61:9. "He it is who has sent his Messenger with guidance and the religion of truth to make it victorious over all religions even though the infidels may resist."
By killing their non-muslim victims, the muhammedans proved that allah was greater than their gods.
Positioning their forefinger straight up is just another form of expressing supremacy over the Orthodox Christian Byzantine posture with three fingers touching (thumb, index and middle) and the rest pointed down.