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Reading a punched card is done by either mechanical, optical or electrical scanning of the inserted punch card. Therefore a pile of cards is put into the hopper and ballasted. After the pile is being loosened by a fan, the cards are read one after the other.
The card puncher punches the code into the cards. Therefor it is fitted with a hopper, from which a card is pulled at a time. This card can be printed on using a keyboard. To punch the holes a stamp is moving from left to right and punches holes from an orthogonal angle. Usually card punchers can only punch 10 holes per second. While errors may occour everytime there is a copy-key that copies the card to a certain column and then one can manually punch in the other holes. The incorrect card has to be removed manually. More sophisticated machines can additionaly print the content to the card as clear text. Some people however can even read punch cards without the clean code printed upon them.
An advancement of the mechanical and electromechanical punchers are programable punch card punchers, that could manipulate cards without having to completely reprint them. Those kind of punchers have still been in use in computing centres till the late 1980's.
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