My take on this issue is this: 
When we hand express, we are applying positive pressure on the ducts to
squeeze the milk out.  
 
When we pump, it is negative pressure or vacuum which removes the milk
from the breast.  
 
When the baby feeds, he does both:  his tongue pressing against the
palate creates positive pressure which moves the milk forward, then the
tongue and lower jaw drop, creating a larger space in the mouth, thus
creating negative pressure/vacuum which forces the milk out..this is why
the baby gets more milk out, but ONLY if he is latched correctly!!!  And
of course the hormones do the rest of the work!!
 It was Barbara Wilson-Clay's lecture which clarified this for me. 
 
You can see it clearly in ultrasounds of a baby removing milk from the
breast.  
 
 This also explains why you can see wonderful jaw excursions when a baby
sucks, but he does not remove milk from the breast..either he has
posterior tongue tie which is restricting contact between tongue and
palate, or he is not creating negative pressure.
 
I agree that the baby is not a machine, but we as LC's have to
understand the mechanics of milk removal in order to help moms.  I am
always stumped by those babies who seem to be doing great on the breast
but are not removing milk.  
 
Esther Grunis, IBCLC, who wished I had paid more attention in physics
class
Lis Maternity Hospital
Tel Aviv, Israel
 

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