The interactive drawing has had the clock automatically started and the principal ellipse is highlighted showing its dynamic movement, which is a cross section of the Earth's ellipsoid that is being taken as the planet rotates.
This dynamic principal ellipse is positioned with its lower turning point permanently fixed to the base center of the static external structure of the building that was defined at the end of the previous section. You can see in the object information panel that the semi minor axis of the principal ellipse is changing over time due to the changing 3D orientation of the Earth's reference ellipsoid from which the cross section is being taken.
It is this dynamic fluctuating ellipse that is used to form much of the internal architecture of the pyramid and which allows the lower northern shaft to be understood.