This helps to virtually double the actual number of channels which can be transmitted. One way of transmitting a signal is in linear polarization, the other by rotating polarization. For the latter, imaging the signal to be transmitted like a screw. In analogue technology, en-crypting a television channel is mostly done by putting the video lines into a different order. Strangely, audio signals are not affected by en-crypting.
Digital transmissions can be en-crypted in a wide choice of ways.
This card contains the code to de-crypt signal. In addition, in most cases, those cards are only distributed to viewers in certain geographical regions.