OK it's one I found online but it's very similar to the one he had last night in Pensicola when the crowd started chanting "Lock her up!". Same expression, shaking his head no, smiling. Now I'm not a body language expert but my read is, that's how a person looks when they know they can't confirm or deny something, so the cognitive dissonance takes over and they smile (acknowledging the truth of the thing), while shaking their head no (as if to say "I can't talk about that...yet"). At least that's my theory!