Actually, this isn't the Illuminati sign at all.
The real Illuminati sign is a closely guarded secret that I will gladly divulge to the world...
Yet in the highest grades of Ordo Illuminati...the following sign is most commonly used...
Notice how the Illuimist demonstrates the technique...
This is interesting George. I remember during my days in the Illuminati at Bohemian Grove they would have a midnight burnt offering of a dozen scrambled eggs. They made the aforementioned sign during the ritual of the Black Pullet and began howling and screaming insanely -- Hail Chicken! But this was an occult watchword for the Combed and Wattled Lord Hell Chicken. Even as it is writ in the Book of the Fowl, "I am the Cock headed digestible Lord of Salads and Soups."
Yet when exposed, the Illuminati were forced to eat Crow, and there was much rejoicing.