I agree with  John Kiernan that its mostly static reduction but I think the
warm breath also expands the block face so you slice off just a skosh more
than your micrometer is set for with less compression and also the heat aids
in adjacent section adherence in the ribbon. Its heartening to see that
huffers anonymous (unanimous?) has such a distinguished roster because it is
an embarrassing posture.  I also resort to the occasional wet thumb rub
followed by a microburst of Histofreeze- no ice cubes near the blade for
me-- only on decals or calcific, hemorrhagic, or keratotic blocks.
I teach all my trainees and coworkers to huff and once they try it, they are
hooked.
Jeff Silverman