When tying in materials, and this applies in particular to bulky materials, you need to handle steps in the diameter of the thread base. If you tie in a bunch of thick deer hair, and trim the butt ends in a straight line, you will probably have a wide diameter zone over the butt ends, dropping in a step to the smaller diameter zone where you have only thread around the shank. This is depicted in the top sketch : A sudden step like this can be managed:  You can leave it as a step, and wind one material on