In order to fully understand nipple confusion, it's necessary to understand how a baby experiences breastfeeding differently than bottle feeding. He is calm and his oxygen levels remain high. On the other hand, bottle feeding is passive. The bottle is poked in the baby's mouth, usually just in the front of the mouth and wiggled around. The flow begins.
Many bottles flow very quickly, rather like a funnel.
Very little pressure must be exerted from the lips to begin the rapid flow. Many times the baby tenses up while drinking a bottle, clenches his fists and learns to hold his tongue back to keep the rapid flow from choking him. He gets very little time to rest. His oxygen levels decrease. His tummy fills rapidly.