Body Language is the study of movement
When someone smiles or laughs you assume they are happy or amused unless their face shows that they are faking it. Either way, you are interpreting what you see, the body language the person is exhibiting.
When someone has bangs doors and thumps the table with their fist, you read the body language and usually, unless there is more body language indicating something else, you assume they are angry.
We all do it. It is natural to us to read body language. Of course, police and psychologists learn more about body language and so are better at picking up the clues from it.
Likewise, when you hold a pen and write, you are exhibiting body language. If you are angry at that time the way you hold and move the pen will differ from if you are relaxed and happy. And so too, the writing that you make will be different at these different times.
The special thing about the body language of writing is that it leaves a mark of how the body moved as the ink makes lines on the page. So when you write when you are blazing mad and when you write when you are calm and relaxed, the lines, or writing on the page will be different.