"Natural sucking" refers to the instinctive action babies have of drawing milk from their mother's breast or a bottle. It is a normal and necessary behavior for infants to receive nourishment. Full definition
A baby's natural sucking rhythm is 40 to 60 cycles per minute (one pull per second or a little less). (healthchecksystems.com)
It's not a good idea to introduce artificial nipples before baby has had time to learn natural sucking motions. (lansinoh.com)
This has led to the development of 2 - phase expression technology which mimics the baby's natural sucking rhythm to express more milk in less time. (amazon.co.uk)