It's just a more gradual taper to the inside of the neck. In theory, if it matched the boattail perfectly, the bullet would sit upright in the case better and start into the case better centered and come out with less runout. I've yet to see that work out reliably, though, because there is seldom an exact match. Short of accomplishing that, though, the more gradual taper assures an easier start into the mouth for any bullet, so it doesn't hurt anything.