watch from 5.30 – 7.02
The cards the guy has used have many tiny lines. The lines on the card are on different angle. At a certain angle lights are able to pass through the card. When light are able to pass the card whatever is behind the card will appear normal but , when the card is turned a different way light will refract differently because the lines on the card; whatever behind the card appears banish . When the light hit the card light cannot pass-through, so light reflect by the lines on the card. When light cannot pass through also means what is a behind the card cannot see. This trick is similar with how polarization work too.