"Rotating pairs" means that you frequently change one half of the pair on a story or epic, to ensure constant diffusion of context about a codebase.

So for example:

Day 1: A & B work on a story.

Day 2: A rotates off. B & C work on the story and complete it. They pick up another story from the same track.

Day 3: B rotates off. C & A work on the new story.

At each point, you preserve and then share context.