Driven by the Geneva, the feed rollers would move the cards the width of one of their 12 rows, then stop. On the read side, that row was under a gang of 80 little bronze wire brushes. On the punch side, the row was over a gang of 80 sharp little steel punches. When the cards stopped a pulse of current went through the brushes. If there was a hole in the card on the read side, current flowed on to one of 80 little solenoids. The solenoid would yank a bell-crank that pushed a punch through the blank card.
The punches withdrew, the Geneva swung its next lobe, and the cards advanced to the next row. Twelve rows per card, 100 c/m. It was quite noisy despite heavy sound insulation on the insides of the covers, a distinct brrruup, brrruup, brrruup overlaid with a general mechanical roar.