JIT runs with minimal buffer stocks, relying on daily or hourly deliveries from trusted suppliers.  As there is no safety net, a faulty shipment of components could bring an entire factory to a halt.  Flow production relies upon holding large amounts of stocks just in case.  JIT rely upon having zero defects whilst flow producers are happy with a quality standard that is just enough.

JIT aims to minimise the costs of holding stocks of raw materials, work in progress and finished goods.  It is often said that in JIT production is pulled through rather than pushed through.  This means that production runs for specific orders.  Stocks are delivered when needed so that almost no buffer stocks are held to guard against production or delivery problems, hence suppliers, workers and machinery must be reliable.  This approach requires several deliveries of stock per day, with the stock being sent directly to the production line for use.

Workers are multi-skilled and are capable of filling in for absent workers.  No spare workers are employed, therefore there is no spare capacity.

Further Reading

Excel - JIT and lean