Milk production is controlled by how often milk is being emptied from the breast. An empty breast makes milk faster than a full breast. The more you empty, the more you make. This is because milk production is being controlled in the breast by a substance called the feedback inhibitor of lactation, or FIL (a polypeptide), which is present in the milk itself. Sometimes one breast stops making milk while the other breast continues (in some cases of slacker boob), for example if a baby nurses on only one side. This is because of the local control of milk production independently within each breast. If milk is not removed, the FIL builds up in the milk and stops the cells from making any more milk. This protects the breast from things like clogged ducts and mastitis. If breast milk is emptied from the breast, the inhibitor is also removed, and making milk resumes. Milk removal can be done by the baby or a pump
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The amount of milk that is produced is determined by the amount of FIL in the milk, which is driven by how much and how often the baby or a pump removes milk from the breast. Emptier breasts have less FIL and make milk faster. Full breasts have more FIL and make milk slower. This mechanism is especially important for continuing to make milk after 11-14 weeks when hormones shift and milk making is completely determined by how much milk is being emptied from the breast.