Look at the angle of Obama’s body compared to the 2 men before him and the man behind him. The others are walking straight in front of that step; Obama is turning his body so that his hand gesture can be seen, even while keeping his head turned as if he was just walking by like the others.
At Pamela Geller’s site somebody posted a video from this event, where he gestured to the others and there was some kind of noise as if there was a comment or laughter. But it seemed like it was a different context than this. I thought I had copied the URL from that youtube video but I guess it didn’t copy. A person could check the positions, etc to see if this is the same instance.
Here's an interesting article in Foreign Affairs which examines the use of the gesture by ISIS...
ISIS Sends a Message--What Gestures Say About Today’s Middle East
Obama knew exactly what he was saying when he made the gesture.
From the article...
When ISIS militants display the sign, to one another or to a photographer, they are actively reaffirming their dedication to that ideology, whose underlying principle demands the destruction of the West. If rank-and-file soldiers are aware of the precise theological implications of their sign -- and it would be no surprise if they are -- that would be a sobering comment on their deep-seated opposition to pluralism.The gesture is equally important for what it means to Westerners, most of whom cannot read Arabic. By raising their index fingers, militants send an easy-to-understand message of the group’s goals of theological supremacy and military hegemony. When potential ISIS recruits in London, New York, or Sydney see the symbol on Twitter, they can grasp the scale of ISIS’ ambitions and its underlying aims. At some visceral level, less-radicalized viewers understand that it means dominance.