Whenever you are talking with anyone, he/she senses subliminal signals about how you feel–sentiments you yourself may not even know you have. When you’re angry or hostile, your hands instinctively close up almost as though you want to make a fist. But of course you don't because that would be antisocial.
A few loose neurons escape however, and your hand (especially your right one if you are right handed) starts to curl. Consciously tell your hands to stay open while talking to hide your negative feelings.