Final answer:
The ISCO of a black hole contract as the black hole transitions from non-rotating to maximum rotation, due to frame-dragging effects which distort spacetime and allow a stable orbit closer to the black hole.
Step-by-step explanation:
If a black hole changes from having no rotation to rotating at its maximum speed, the Innermost Stable Circular Orbit (ISCO) contracts. This is because the rotation of a black hole affects the spacetime around it, causing the region where the stable orbit can exist to move closer to the black hole. In a non-rotating black hole, the ISCO is located at a larger radius than in a maximally rotating black hole. When the black hole begins to spin, it drags the spacetime around with it (an effect known as frame-dragging), which results in a smaller orbit for the ISCO. This is an example of the conservation of angular momentum, similar to a figure skater pulling their arms in to spin faster.