The oxytocin is released into the blood stream and then hits receptors.  
Babies don't get colostrum by just sucking.  The breast (cells around aveoli) have to respond by squeezing the aveoli and pushing colostrum out.

Cleaning out a box of "stuff", my current project since I have almost been 2 years here, LOL.
Found a quiz and question 3 just jumped out at me!
3. Signs of let down include:
a. milk leaking or dripping
b. drowsiness
c. gush of vaginal bleeding or cramps
d. pins and needles sensation in the breast.

You could pick 1. a,c,d  2. a,d    c 3. all  :-)

Back to the piles of paper!  Just found something from 1996!  Pat in SNJ

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