When White is komaster saves the
stones. Later,
-
is sente. (Somewhat sneakily, Wa instead is also sente. ;-)) Then a, b, c, and d are miai, played as dame or ko threats.
That leaves behind a large throw-in ko at . But Black takes first with
, forcing
. Now
saves his stones. White still has a sente ko with
- Ba. (Since Black is komaster, White will avoid this ko.)