Xuanxuan Qijing Problem 1 / Solution

Main line  

Why would black choose here? See the following variations.

variation 'a'  

If black plays this instead of in the previous diagram, white replies with through . Next white plays c to get out.

variation 'b'  

If black plays here instead of in the main line, the sequence in the previous diagram still works.

 

Black can't block with in the main line, otherwise white can just push twice and make double atari to break out. If black blocks with in the main line, white can break out in a similar way.