A simple version of this (not the exact experiment) is to take three polarizing filters. Put one down, and see it reduces the amount of light passing through. Put a second one down, in line with the first, then rotate it to 90 degrees off. You will observe the light dim until the second filter is completely dark, blocking all the light which passed the first filter. Finally, take the third filter, and place it between the first two, at 45 degrees off, and suddenly light will pass through all three.

This very clearly demonstrates that each filter reshapes the wave's polarization, not to a single orientation, but to a narrow band.