1. What is body language?
A type of non-verbal communication in which physical behaviour (as opposed to words) are used to express or convey information.
It includes facial expressions, body posture, movements and gestures, eye movement and eye contact, touch and the use of space. skin tone, even people's breathing rate and perspiration
It is also known as kinesics.
Additionally, the tone of voice, the rate of speech and the pitch of the voice all add to the words that are being used.
It is anything someone does to which someone else assigns meaning. Not all of the "signals" a person sends are intentional and often they are "not picked up" or misinterpreted. Nonverbal behaviour is complex, subtle, and multichannel.
It is important to recognize that body language may vary between individuals, and between different cultures and nationalities. 1

2. How does it work?
“You can't hide your lyin’ eyes," the Eagles cautioned in their mid-1970s Top 40 hit. And they were right.
Humans almost certainly relied on interpreting one another's body movements, posture and facial expressions long before they had words to express what they were sensing.
Some body language may be culture-specific:
In Europe a smile may indicate happiness, while in Asia, it may indicate agreement.
And while we may see eye contact as a gesture of attentiveness or self-assurance, in some Asian and African countries, it's perceived as disrespectful.
In some countries, it's common to emphasize one's words by waving the arms, but Japanese speakers avoid doing that because in their country it's deemed impolite.
Experts say that by becoming aware of your body language, you can train yourself to control it, and even consciously harness it to make your communication more effective. 2

3. The [so-called] 7%-38%-55% Rule
Prof Albert Mehrabian, Professor Emeritus of Psychology UCLA, is known for his pioneering work in the field of nonverbal communication (body language). His work helped identify nonverbal ways in which one conveys like-dislike, power and leadership, discomfort and insecurity, social attractiveness, persuasiveness, and ways to detect when others are being deceptive in communication.
According to Mehrabian, three elements account differently for our liking for the person who puts forward a message concerning their feelings:
words account for 7%,
tone of voice accounts for 38%, and
body language accounts for 55% of the liking.
For effective communication, these three parts of the message need to support each other (to be "congruent”) for the receiver of the message to ‘hear’ it.
The following example should help illustrate incongruence in verbal and non-verbal communication.
Verbal: "I do not have a problem with you!"
Non-verbal: person avoids eye-contact, looks anxious, has a closed body language, etc.
It becomes more likely that the receiver will trust the predominant form of communication, which to Mehrabian's findings is the non-verbal impact of tone+gestures (38% + 55%), rather than the literal meaning of the words (7%). This is known as "the 7%-38%-55% Rule” and has been much misinterpreted. 3

4. Some examples of body language
Disengagement: 
Heads down
Eyes glazed, or gazing at something else
Hands may be picking at clothes, fiddling with pens, writing or doodling
Sitting slumped in their chairs.
First Impressions and confidence:
Posture – standing tall with shoulders back.
Eye contact – solid with a "smiling" face.
Gestures with hands and arms – purposeful and deliberate.
Speech – slow and clear.
Tone of voice – moderate to low.
Reflecting:
Eyes look away and return to engage contact only when answering.
Finger stroking on chin.
Hand to cheek.
Head tilted with eyes looking up.
Lying:
Little or no eye contact, or rapid eye movements, with pupils constricted
Hand or fingers in front of mouth when speaking
Body physically turned away
Breathing rate increases
Complexion changes such as in colour; red in face or neck area.
Perspiration increases.
Voice changes such as change in pitch, stammering, throat clearing.
Defensiveness:
Hand/arm gestures are small and close to body
Minimal facial expressions
Body physically turned away
Arms crossed in front of body
Eyes maintain little contact, or are downcast 4

5. How might it be useful in coaching?
Awareness and control of body language can be useful to the coach – but also to the coachee:
A good coach monitors body language and tone of voice to check that these things are congruent with what the coachee is saying
Three ideas from NLP could behelpful:
Pacing, that is, matching the coachee’s speed and volume of speech
Mirroring which is when you match the body language of the coachee (carefully and subtly)
When these have helped you establish rapport you can then use Leading - changing your own non-verbal communication bringing the coachee with you
There is lots of information on the web about understanding body language e.g. TED talks: 5