You put the semi-tillered bow on a form, or use a string to pull it into reflex, and then either slowly use a heat gun to darken the limbs (holding it 3-6" away) so the entire limb is heated through, or hold it upside down over a hot plate/bed of coals.
It increases the compression strength of the belly, making it easier to hold reflex or lower mass. If done properly it can, without even inducing reflex, increase performance about 6 fps. It is like a sinew backing only more reliable and less expensive