JIT (Just-in-time) principle came to be known before the actual Lean thinking as one of the Japanese production philosophies driving principle. In Finnish is also used JOT (Juuri Oikeaan Tarpeeseen) expression that describes the principle well: materials are produced, transported and transferred based on a real need. The real need comes from customer demand. Narrowly defined, JIT is practically the same as ”pull control”.
The idea behind pull control is that inventories cause costs and hide problems in the process and should therefore be minimized. In Lean thinking Pull control is a production control method, which is based the pace of customer demand, and in which the amount of inventories and WIP (work in process) is limited. Products and semi-finished products are manufactured and moved forward only if there is a need, the next step in the chain requests for it (downstream or to customer). The next step in the chain, therefore, is a customer, whose need controls the previous step.
Pull control is the easiest to implement in material flows or their parts, in which need is reasonably steady and supplements are fast. Pull control is more challenging in situations where, for example, demand varies strongly or supplement times are long and possibly variable: for example, far situated supplier.
Push control describes the principle in which the material flow is not directly pulled by customer need, but actions in each step are based on pre-made plan, for example, production plan. Usually in push control, Material Requirements Planning is used to manage the whole flow. There is no upper limit for work in process (WIP) or inventories.