I know that "technically" there is no such thing as an empty lactating breast,
but realistically, if the baby nurses long enough, the breast gets empty
enough for the baby to want to switch sides if he's still hungry. Sure, there
might be a drop coming after several sucks, but if the baby wants more, this
small amount would be frustrating. It would be like using a straw to get that
last bit of chocolate shake from the bottom of the cup, when what you really
want is another shake!
However, other babies are content to nurse on an almost empty breast and they
don't want the second side at all; in fact, they get angry when it's offered
to them. I remember when I was working through overactive letdown with baby
#2, thinking "I think he really just wants to suck on an empty breast!" The
problem was, that I had such an over supply (from frequent switching) that I
did not ever HAVE a breast that was empty. Both were VERY full all the time.
The trick was to let him finish the first side (breast feels very soft and
baby's swallows really spread apart). The first day it took him 12 hours to
"finish" the first breast. Meanwhile, I hand expressed the full side to
comfort.

I try to take a simple approach with moms:
-Offer one side.
-Let the baby stay latched until he falls asleep or sucking slows down a lot.
-Burp the baby if you want.
-Try the second side but don't worry about it if baby doesn't want it.
-Be aware that the baby might want to stay on the "empty" side.

Mary Ann Whitesell, IBCLC