Mahavey,
It's redirection. With standard dies, the familiar course of events is a resized case leaves a bit of head clearance, and firing stretches the case at the pressure ring to expand to fill that space. When the case is resized again, entering the sizing die first contacts the sides of the fired case, squeezing them inward and elongating the case further. In this narrower-but-longer condition, as the shoulder of the case reaches the shoulder in the die, the press ram pushing the case deeper into the die causes the shoulder of the case to be extruded by the die shoulder, turning the top edge of the case wall into the bottom edge of the shoulder, and the top of the original shoulder sliding inward and mostly up into the neck, but partly inward to form the interior "donut", while the rest grows the neck length and thereby causes overall case growth.
In the X-Die, two things are different. One is that instead of the neck portion of the die being open at the end, it has a sharp shoulder that limits how long the case can get. The second is that it has a mandrel as the decapping pin stem that prevents the excess brass from flowing inward or forming an internal donut. So, Given those barriers to growth, where does the excess extruding brass go? The only place left is down into the powder space. So, while I don't own one of these dies, my expectation is that sectioning a case that has been resized in one for a number of cycles would show a sort of inverted lip or collar at the bottom of the neck where it meets the top of the shoulder, protruding downward into the case. Rather than an internal donut narrowing the neck diameter, it would have a downward donut that slightly extends the internal neck length. It's the only place left for the extruded brass to go.