When user testing how do you record positive or negative body language? Do you use observed shifts in body language or facial expressions to consciously probe further about what the user thinks or feels about the product or task they are completing?
For recording body language during usability testing sessions I've found the behaviors listed in this coding form from measuring UX (pdf) quite useful. Things like frowning, fidgeting, or leaning in close to the screen are often as meaningful as verbal remarks.
Observing body language and facial expressions requires focused attention, it is really easy to miss a frown or a quick smile. If you have the luxury of having an extra observer present, they can mark occurring behaviors and expressions during the test. The facilitator already has enough to do during the test, but it can be helpful when he is alert to nonverbal signs.
Why is this useful? When you're not just searching for issues, but also trying to achieve a particular emotional experience, statements like 'most users reacted to this page with surprise' can be quite valuable. Besides that, video recordings of nonverbal behavior work wonders in selling/presenting the results of your user testing: showing angry and frustrated reactions to an issue really helps in getting the message across to the stakeholders.
I would be careful about making too many judgements based on body language unless you have experience in this. In analysing interviews there are various techniques you can use, such as counting the frequency of certain gestures or facial expressions, but you have to be sure you are making the right interpretation and be consistent on what consitutes a certain gesture.
Apart from for very strong expressions/body language I wouldn't record them when analysing the video, but they can be used as a guide as to when the facilitator should interact with the user during the session.
Non-verbal reactions help you contextualize verbal feedback. They also help you direct the user's discussion of the system under test. Alone, non-verbal signals are very easy to misinterpret, which is why you should follow up with the user to understand what is going on.
It is very powerful to name the behavior, and ask the user to expand. Just give them enough conversational space to disagree with your observation if they need to: "It looked like you might have grimaced there. Were you reacting to something?" "Oh, I wasn't grimacing. [grimace] It's just that this scroll bar is driving me nuts..."
I generally don't write them down specifically--they provide connotation to other notes. But if I were being a pure observer, rather than the facilitator (and therefore unable to ask follow-up questions), I would write down my impression but flag it as interpretation. "User seemed to be confused by dialog box." "User hesitated and grimaced when asked to sort items."