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I suggest EVERYONE buy and read this book
The Definitive Book of Body Language. (by Allan & Barbara Pease)
This will answer your very question. Lots of hand motions while talking denotes that you are nervous or anxious.
That is a great book I completely agree. Another great book is Love Signals, which I found was more helpful from a dating perspective. I think Body Language is by far the best book to read as far as observing human behavior. I can read people a lot better 2 days after I read it I won a poker tournament.
I would recommend that book followed up by Love Signals by David Givens. Love Signals shows everything in a animalistic nature. How we have not evolved at the speed of our culture and that we still are very primitive in that we send mating signals, why we are initially attracted to girls. It also shows how to build rapport without saying a word.
Hand motions are very good to have and use correctly. You want to have open palms when talking to a girl. Also small things will help like copying her hand gestures. Basically show safe and non threatening movements.