The system itself works by having several predetermined indicators set, which are called “cards,” that will tell the employees when something has to be done. For example, if more inventory needs to be produced when the warehouse is down to 20 units, then 20 units will be the card for that system.
When 20 units is reached, the card will be turned in to the manufacturer and more units will be produced. Likewise, if an upper card is set, production will stop until the lower limit is reached. This keeps inventory costs down and management happy.