A Push System produces work based on forecasts or schedules regardless of whether the downstream process actually needs it. Each step pushes its output forward as soon as it is complete. It is the traditional default in manufacturing, and it is the source of most of the waste that lean tries to eliminate.
A central plan tells each step what to produce and when. Steps run to their own schedule, based on forecasts of what the next step will need. Completed work is pushed downstream to be worked on when the next step is ready. Inventory accumulates between steps as a buffer against imbalances in the plan.
Forecasts are almost always wrong. Push systems either produce what wasn't needed (creating inventory and overproduction) or fail to produce what was needed (creating shortages and expediting). Inventory between steps hides problems, absorbs capacity, and inflates lead time. When demand changes, the whole plan has to be reworked and the in-flight inventory becomes obsolete.
A pull system reverses the logic. Nothing is produced until the downstream step signals that it needs more, typically via a Kanban card. Upstream steps produce only what is pulled, in the quantities pulled, when it is pulled. Inventory shrinks, lead time drops, and the whole system self-regulates around actual demand instead of forecast demand.
Push is what factories default to when they are not deliberate about the alternative. Moving from push to pull is one of the largest lean transformations a factory can undertake, and one of the most rewarding, because most of the eight wastes shrink together the moment production is triggered by real demand instead of by a plan.
