The cannealure, of course is of slightly less diameter than the bullet, thus allowing room for the mouth to crimp in to.
The difference between the taper crimp die and the roll crimp die: The opening at the base of the taper crimp die, starts out large (relatively speaking) and gradually narrows, simply forcing the case to squeeze the bullet.
The roll crimp die "narrows" in just a few thousands of an inch, forcing the mouth to "roll" inth the bullet. If adjusted down too far, the case will buckle.
I hope that one, my explantion makes sense, and two, that it answers your question. It sure would be a lot easier to draw you a picture.