This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
Once the basics of getting around a corner are developed, doing it
fast involves careful use of the brakes. Besides knowing how steeply
to lean in curves, understanding braking makes the difference between
the average and the fast rider. When approaching a curve with good
traction, the front brake can be used almost exclusively, because it
is capable of slowing the bicycle so rapidly that nearly all weight
transfers to the front wheel, at which point the rear brake is nearly
useless. Once in the curve, more and more traction is used to resist
lateral slip as the lean angle increases, but that does not mean the
brakes cannot be used. When banked over, braking should be done with
both brakes, because now neither wheel has much traction to spare and
with lighter braking, weight transfers diminishes. A feel for how
hard the front brake must be applied for rear wheel lift-off, can be
developed at low speed.