Natural micromovements are those which are most common to everyone, such as shaking with fear, blushing with embarrassment, crossing the arms when one is judgemental, etc. It is the mnoitor's duty, as he/she works with viiewers, to learn their natural micromovements in order to understand the various moods, site-reactions, etc. which do not get directly expressed in the session transcript. The monitor makes nots of these micromovements throughout the session, and adds those notes to the session transcript that is given to the analyst. In that way, the analyst can better judge the validity, importance, and purity of the viewer's perceptions.
A monitor who wants to learn more about natural micromovements should get a good book on body language and study it thoroughly.
See also, "micromovements" and "CRV micromovements".