If you're going to answer this question I'm going to assume you already know what a micro expression is. My question is when I'm talking with a girl, sometimes I'll say or do something and her eyes will open very wide, exposing the whites of her eyes both above and below the pupil. It seems at times I've said something very impressive, other times I've said something which may cause concern for the girl. I don't quite understand if this is a good indication from the girl or a negative. Have I touched on touchy subject for her? Or is she impressed? Perhaps this micro expression response could be either way and simply needs to be interpreted in the scenario, but I can't seem to always read which one she's feeling when I first begin to talk to a new target. Thanks guys!

A rule in reading body language is supposed to be read the body language in chunks, isn't it?

1 gesture can mean a lot of things. But when its combined with other gestures that happen at the same time it limits the possibilities.

Microgestures(normally pacifiers) and microexpressions are an exception to the rule because they are involuntary, once you have a large understanding of body language then you start picking up on the things no one notices. Example of Microgestures: When someone touches their ear at the same time, or when someone rubs the back of their neck. They are showing their discomfort with something that just happened(it may even be something they thought about).

Microexpressions are very special and have a direct link to the brain, often times you can name an emotion before someone even realizes they feel it. Microexpressions show up everywhere if you know what you are looking for, but they are often so short that only people trained in microexpressions will pick them up. There is the occasional natural that has the instincts to read them but more often then not it requires training.

The lesson is Microgestures and Microexpressions are involuntary reactions, it takes a great understanding of body language to get to that point but they are quite trustworthy when you align it with timing.

When it comes to a lot of things yes chunks make it more reliable, but when it comes to reading statements within statements, or reading moments of emotions the most reliable thing is microexpressions and microgestures.

Hmm.. I thought microgestures (Cool to have a name for it) was pretty obvious.
It really does sound how Poeticlyskuac says, I'd take it as suprise and try and work the reason from trying to pick up on other involuntary movements.

Maybe rather than trying to work out what that gesture means, use it as a signal to pick up on any oncoming gestures.