Originally posted by apathist
Apparently it is a piezoelectric effect modified by a strain gradient (wiki). I don't know what a strain gradient is.
Gradient is just how the stress varies across a depth, so when bent, the inside curve may have more stress than the outside, which can be seen by laser interferometry.
Here are a bunch of images where you can see the distortions and stresses with this technique:
These devices can tell how well a telescope mirror is ground, and whether there is strain on the device.