Sheer speculation toggle ON.....
Since the poster indicated that they were making whole wheat bread, I think
what they might be referring to is the mixture and fermentation of the
starter for the bread -- this usually contains a small portion of the flour
required for the recipe. The starter is left to "rise and bubble" for a
period of time, after which the main bread recipe is made using the
starter. Whole wheat breads tend to be heavier than other types, so this
probably gives it the equivalent of an extra rise, making the loaves
lighter and fluffier.
Many of the starter-type bread recipes that I'm familiar with indicate that
it is satisfactory to leave the starter as long as 24 hours, sometimes in a
cool place (e.g., refrigerator) and at other times on the countertop at
warmer temperatures -- depends on the bread. My experience is that this
often yields a lovely flavor to the bread in addition to making it lighter
-- I have a heckuva ciabbatta recipe that uses this method, although it's
not whole wheat (see my recipe page at
http://www.geocities.com/Heartland/8098/recipes.html to find the ciabbatta
recipe and quite a few of my other favorite bread bread recipes). I've
left the starter for my ciabbatta in the fridge for as long as a week
before I got around to making it, and I've found that the longer it is left
the better the flavor!
Sheer speculation toggle OFF.....
Joan