Eye crunch is the natural effect that occurs in your eyes when you are looking at something. Just like the parts of your mouth work differently when you are using them, the eye works in different ways depending on how it is being used.
Since we are using our eyes to see constantly when we are awake, we tend to overlook the fact that we do use them in several different ways. Eye crunch is the way the internal parts of our eye react and send different signals to the brain depending on how the eye is used and the different parts of the eye that are being strained and utilized.
The biggest difference in how we use our eyes is when we are viewing something that is up close as opposed to something which is in the distance. The eye crunch in each of these circumstances is different as different parts of our eyes have different parts to play and thus different strains when looking far away or looking up close. Another example of a time when eye crunch plays a big part in how our eyes react to what we are doing is when we are reading.