My understanding of the fluid dynamics of the milk ejection reflex (let-down)
is that the milk fluid build up causes positive pressure inside the breast.
 Baby creates a negative pressure space inside the oral cavity by sucking.
 The MER creates a spurt or extra push of positive pressure behind the milk
in the breast increasing the pressure differential between the breast and the
negative space in baby's mouth.  This helps the fluid move more easily from
the positive to the negative space.  Sucking then maintains the flow.  At
some point, another MER may occur to assist this process.  This is just what
I've absorbed from reading.  I also would be interested in more informed info
on this.
Barbara Wilson-clay, BSE, IBCLC
Austin, Tx  priv. pract.