Looking at the picture of the "pull type" part might help:
The small cylinder with the blue cap is the "external changeover valve".
The master cylinder has two chambers, and one has a larger diameter than the other. In hydraulics, the difference in cylinder size is what gives you a mechanical advantage. Initially when the large cylinder is active and high pressure is not needed, a larger amount of fluid is pushed into the system. Once the pressure rises (as the brake pads contact the disk and tighten up), the external valve changes from the primary (large) chamber to the secondary (small) chamber. The smaller chamber provides a greater mechanical advantage, and the smaller amount of fluid is fine because the brakes are already tight.
Once the secondary chamber is activated, the large chamber is not under any pressure at all. Furthermore, if you continue reading their description, the primary chamber isn't activated again until the driver completely releases pressure on the pedal to prevent the switchover from happening rapidly (in the middle of a corner for instance).
Lets compare this with a typical master cylinder from a passenger vehicle:
Here we see two chambers that are compressed by two plungers that connect to the same rod. This provides hydraulic power for two circuits at the same time, like the front and rear brakes.
The differential bore model looks very similar inside except the "primary" is larger than the "secondary", and both of the outputs are controlled by the external changeover valve. When braking begins, the output of the secondary cylinder is vented back into the reservoir so it moves freely. The primary output is directed to the brakes. Once pressure increases, the changeover valve switches so that the output of the primary is directed to the reservoir and it moves freely. The secondary output is now directed to the brakes providing greater force.
Note that since AP doesn't provide any diagrams for the internal, patented workings of their cylinder, the implementation details could vary.