Any a play is answered by either b; and vice versa. Thus Black secures eyes on the left group. So White attacks on the right. It looks like Black won't have enough eye space. But the cutting point (circled) proves fatal to White's plans.
threatens to capture and thus connect the groups, so White must fill.
makes the necessary eyes.
Any other inside play, or a play at the squared point, is met by , which gives atari and also settles the eye space into a bent four. The circled points are miai to divide the space.
If White fills, Black can still claim enough eye space this way. In this variation White can't play at
because of shortage of liberties, and Black captures to make two eyes.
Similarly here.