Your best reference about this would be Mike Woolridge's chapter in Dettwyler
and Stuart-Macadam's book.  Here's how I understand it.

Milk is being made at a steady rate all the time, so the breast is
accumulating milk at a steady rate.  A breast that hasn't been nursed for
awhile is fuller than one recently nursed, and the milk that trickles toward
the front is the lower-fat milk.  So when the baby latches on, he gets the
lower fat milk first, and as the breast gets relatively emptier, the higher
fat milk is squeezed toward the front of the breast by the basket cells.  So
the relatively emply breast (relative to itself when fuller) gives higher fat
milk.

If the baby takes a break for ten minutes, then ten minutes' worth of milk
will accumulate in the breast.  So the baby has to "get past" only that ten
minutes' output before he can go on with the job of getting at the richer
hind milk.  If you figure 24 ounces a day as the average milk production,
that's a half ounce (15 cc) per hour per breast, so ten minutes wouldn't mean
a whole lot in the way of milk accumulation---a couple of ccs is all.  If he
takes an hour break, then there's time to collect 15 ccs.

The longer the interval between feeds, the lower the fat content of the fore
milk when he does latch on.  Conversely, the shorter the interval between
feeds, the higher the overall fat content of the milk.  So those !Kung babies
who nurse for two minutes every quarter-hour are keeping the fat level
relatively high, especially if they do the same breast several times in a
row.

Of course, the longer the baby spends on the breast (nursing effectively),
the emptier the breast becomes, and the higher the fat content of the milk.
 So nursing long and strong is another way a baby can access the high-fat
milk.

Remember that the !Kung babies slept with their mothers at night, so if they
needed long nursings to balance their nutrient intake in some way, they had
the opportunity to do it then.

It's my hunch that "growth spurts" may be a baby's way of adjusting the fat
content of the milk to boost his caloric intake, rather than---or in addition
to---his way of increasing his volume intake.  Women's daily average milk
output rises from 750 to 800+ cc per day over the course of lactation, but
who's measuring the average caloric contect of the milk?  Does that increase?
 And are there other changes taking place as the baby grows---more efficient
absorbtion of nutrients as an effect of epidermal growth factor in the gut?

One thing the snacking babies do for their moms that the bolus-feeding don't
do as well is to keep mom's prolactin level higher.  And that reduces the
likelihood of ovulation, which is surely in the baby's best interest!

I find it marvelous that the baby turns out to be in charge of Everything!
 The amount of milk, the amount of fat, and the spacing of his next sibling.
 Ain't it great to be a mammal!

Chris