It actually works by using polarized light. LCD monitors (see Wikipedia article here) have a polarized filter on them and the hack shows the removal of this filter and placing them on the glasses. If you have a set of polarized sunglasses or a polarizing camera lens, you may notice that you can see what is on the screen in one orientation, but rotate it 90 degrees, and you cannot see it anymore.
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"Tongue tied and twisted, just an earth-bound misfit, I" - Pink Floyd
(and then the tower cleared me for take off)