The first polarizing filter limits the vibration of light waves to one plane—that is, it polarizes the light.
The white light from the overhead projector is made up of light of all colors. The plastic breaks the light waves that make up each color into two perpendicularly polarized waves. These two waves travel through the plastic at different speeds, which are determined by the light’s color. When the two waves meet and recombine, they produce a polarization unique to that color.
The direction of polarization determines whether light of a certain color can pass through the second polarizing filter. If the new direction of polarization lines up with the second filter, light of that color passes through the filter and you see it. If the new direction of polarization does not line up with the second filter, light of that color is blocked. By rotating the filter, you can let different colors pass through, and the colors you observe will change.
Stressing the plastic alters its structure, which affects how rapidly light of different polarizations travels through the plastic. Where colored patterns change rapidly, stress is high. Where colored regions are spread out and change gradually, stress is low. Sharp corners, or areas that have been cut or stamped, are usually areas of stress concentration. Changing the stresses in the plastic will change the color patterns in the plastic.
Stress patterns and concentrations like the ones visible in the plastic are also present in your bones, as they flex under the daily loads imposed upon them.