In just in time production system, when is an upstream production triggered?

Explanation

Along with Jidoka, Just-in-time (JIT) production is one of the pillars of the two Toyota Production System. It is a production method that fundamentally changed the way large-scale production oc-curred in the 20th century, and is the basis for Lean Manufacturing (Lean for short), which is the school of thought many modern companies have modeled themselves after.

JIT production is often called a pull system because instead of using traditional push production where scheduling is done based on historical data and demand forecasting, production is scheduled based on actual customer orders. Rather than predicting demands from customers, the JIT method requires that actual customer demand exists. Production doesn't begin before an order triggers it. This system not only reduces the amount of extra inventory, but also reduces the amount of work in progress at one time.