The Turn
At 1.3 seconds, Obama begins bringing his left shoulder around to follow the turn he initiated with his head. At this point Woman B's ass comes into his field of vision. But Obama's head is still in mid-turn--he's bringing it around to see Woman A, whom he intends to help down the stair. Woman A begins preparing for this by moving the item she'd been holding in her left hand--a credential? A program?--to her right hand, freeing up her left for Obama's. This indicates that some sort of communication or coordination, verbal or otherwise, had transpired between the two to indicate to Woman A that Obama intended to help her down the step.