While frequent nursing or breastpumping are essential to establishing a solid milk production for the mother, they are not the key elements for triggering the initial milk production in the mother's body.

During pregnancy, several months before the birth of the baby, the mother's body prepares for milk production of the breasts by a host of different kinds of hormons such as prolactin, growth hormone, insulin, thyroid hormone. Many of these kinds of hormons are produced by the mother's body, others are from the placenta. However, there is no actual milk that is produced until 30 to 72 hours after the baby is born.

The reason is that the styroid hormons produces by the placenta actually block milk production hormon prolactin from circulating up to the breasts for establishing milk production, thus no milk will be produced unless all the placenta tissues are removed.

Therefore, the true inital milk production is established by removing of the placenta, not the newborn suckling. However, after that initial blockage removal, the 'supply and demand law" kicks in. More nursing or breast pumping means milk is in high demand and vice versa. Therefore, frequent nursing and breastpumping is crucial to maintaining milk production in the nursing mother's body.

Further reading:

Non-nutritional factors affecting milk production

How Milk is Made

kellymom.com : How does milk production work?