When the Cerrick reaches the World's End, Cyrus responds by careening the ship - up is down, now - so as not to fall off the edge.* Call it leaping the scales, bending at the Bones, or bricking the king's window; they have to punch through a mirror-barrier.
Who knows? Perhaps the Eye of Magnus is Topal's lost lightship the Niben? Or at least it's Waystone. Navigational directives may have been conveyed to the piloting synesthete, from Crystal like Law, by varying wavelengths on the explorer's refractory path.
Point being, is time curved around the continents, and can one travel back by going forwards; or going back to future step? Does the smaller Pyan-Sum-Thras "gear" drive the larger tectonic gear Yoku-Tam-Aka?
*Lord Vivec's Sword-Meeting