O'PEEP'S PULL EXPLANATION
Are you hungry? Your body is telling you some calories are lacking? Communicating that something was lacking - This is Pull!
You are eating according to a diet plan? You are always eating everything that you are being served? You are triggered by material or by information on what to do - That is Push!
There is one simple principle of a system that works with pull: The demand is triggering a system to deliver. Production gets started or action gets initiated only if there is a customer need. This avoids excess inventories and overproduction.
Pull systems make sense if continuous flow is not possible, e.g. because many product variants have to be produced, because of frequent machine breakdowns, or external suppliers that don't guarantee continuous shipments. Pull requires quick and frequent replenishment (small batch sizes, quick changeovers) to be effective without too much inventory.
A tool commonly used to develop a Pull system are Kanbans.