Cell wall extension- shearing units??

[MelJayne]

I am currently reading about the plant cell wall and it's expansion (for an assignment which happens to be in tomorrow!) and have come across a term i cannot find the explanation for- shearing units. Any help would be greatly appreciated (even if past the deadline, i would still like to know). Here's the section of text;

"Wall extension commonly exhibits a yield threshold stress, below which walls do not extend. Although its molecular nature is not well understood, one way to think about the yield threshold is to consider that it depends on the size of the shearing units within the cell wall – the smaller the size, the lower the threshold."