I can't find any transcription of this TEDx Talk so you're just going to have to watch it. It's worth it, so just do it.
We don't always realise how much of our thoughts and emotions come through in our body language.
When you start paying attention to your gestures and expressions you start recognising the thoughts and emotions leaking out of the cracks and showing in your outward behaviour. You can use your body language as a reference for yourself to test whether you are clear about a specific point within you (ei you are not having thoughts or experiencing emotions about whatever the point is that you may or may not be fully aware of). When it comes to body language, you'll probably get the most information when you're communicating with someone else, but you can also test your awareness and the nature of your body language when you're alone - have you allowed a bad mood to get you down and let your body go into a slump? Are you feeling confident about something? Are your thoughts spinning around in your mind? Your body language can give you insight to these things and more.
Facial expressions are also telling of what you're thinking and how you're reacting within your mind. Most people have not developed the skill of controlling their body language or facial expressions, because of this you can be relatively certain that you are getting some pretty honest feedback from observing yourself.
It is interesting to film yourself in action and then watch the video to see, from an 'outside perspective' exactly what the hell you're actually doing when you're (for example) having a conversation with someone. You will probably see your opinions of a person come through quite clearly.
The goal is not to hide your body language and expressions so as to become a better liar - the goal is to stop the thoughts and emotions that are behind certain behaviours and expressions. Take the following as an example: You are discussing a topic with a person you know well, but the topic starts shifting into a conflict zone where either one or both of you are becoming agitated/irritated/whatever - the best thing is not to hide your state of reaction better, but to change your approach to the situation so that the conflict doesn't happen. You want to develop your communication skills and your openness to considering things outside your 'comfort zone' so that you can effectively direct a social situation without having to resort to all sorts of negative experiences and displays. This would mean letting go of your desire to be right and your desire to be in control.
The other aspect that Allan discusses which I have also written about in a previous post, is the point of 'fake it till you make it'. Developing your body language and expressions with awareness actually imprints those ideas, principles, ideals and 'thought patterns' into you - so if you are pretending to be confident in your body language you actually start feeling more confident and with time your entire way of thinking and of being changes to incorporate this confidence and it becomes a part of your natural expression .Obviously this is only to a certain degree. Continuing with the example of confidence, if you have spent most of your life judging yourself and generally being nasty to yourself you will still need to address the root of that habit - pretending to be confident is not going to conquer your demons of old - but it certainly will make it easier.