The king has reached the corner the bishop does not control. The W manoeuvre marches it along the edge to the right corner: the knight zigzags, the bishop seals, the king shadows.
Stage two of the drill. Black's king is on a8, a light square, and White's bishop is dark-squared, so a8 is the wrong corner. White must escort the king along the eighth rank to h8 (or down the a-file to a1); the standard is the back rank.