King on d1: Queenside

Same position, but now queenside — and there's a funny twist here! The king will step from d1 just one square, to c1, while the rook runs over from a1 and lands… exactly on d1, where the king was just standing! It's like the pieces swap houses. It confused me at first, and then I got it: they're simply going to their lawful squares, c1 and d1. Drag the king onto the rook on a1!