The important factor is a pressure differential between the inside and
outside of the breast sufficient to overcome the resistance of the
nipple muscles to milk flow. There are two ways to enhance this pressure
differential and remove milk - apply vacuum to the nipple (the way
babies mostly work) and apply greater positive pressure to the inside of
the breast (which is how manual expression works). Since the breast is
pliable, pressing the outside makes the inside smaller too, increasing
the pressure in the ducts. Mother's milk ejection reflex increases the
pressure inside the breast, so there is more of a differential from
inside to outside, enhancing the baby's ability to get milk out with
vacuum (negative pressure) or the mother's ability to get it out with
manual expression (positive pressure). Einer Egnell worked this all out
(complete with numbers for these pressures) and published it in the 1950's.

Catherine Watson Genna, BS, IBCLC  NYC

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