I suspect you are referring to the practice still common
with many artists today who shape the brush tip with
their saliva after it has been cleaned and before putting
it away during uses. Not a smart thing to do in spite of
the fact the brush is supposed to be clean. I've watched
watercolorists do this particularly, since the fine sable
bristles are more conducive to this kind of shaping.
For those who need to shape their brush tips there are
neutral water-soluble mediums that work well. Gum Arabic
is the vehicle in watercolor paints and does a great job
of glueing the bristles between uses. It can be purchased
at any printmaking supply store since it is widely used
in printmaking. Getting it in less than gallon quantities
may be a problem so sharing the solution with other artists
is the solution.