Walkie Talkie Seven
This shape can be turned into a throw-in ko. White has two occasions to turn it into seki. The more outside liberties, the less likely the ko will be played (and thus the more likely White makes it seki in the end).
The vital point of this shape obviously is . After
, any move by Black other than a results in death in gote. It is no use for Black to throw in at a either, as it makes a ten thousand year ko.
If Black connects at a, his eyeshape is reduced to a pyramid four, and therefore Black is dead. Playing on either of the points b allows White to capture at a and this puts Black in atari. Unless Black is komaster with big ko threats, he is unlikely to want to play on either point b.
Letting White capture at a and connect at turns this a position in a seki.
( and
= tenuki)
If Black plays elsewhere instead of starting the ten-thousand year ko, White can try to capture the corner by playing out the ko. However, White will need two more ko threats than Black, and Black will get to play four moves elsewhere in exchange for the corner (,
,
, and
). If White doesn't have enough ko threats, an option is to back down by playing
at
in the first diagram to make a seki, but Black would still get to play three moves elsewhere (
,
, plus one more move elsewhere after White makes seki).
Hence, it would be difficult for White to win the ko without being komaster.
This way the position becomes a direct ko, which is an inferior result for Black. White can finish the ko by filling at - it would be a mistake to play one of the other inside liberties instead.
Although the situation shown here is perhaps unlikely to occur in real games, it is of theoretical importance. If Black's group has three or more external liberties, White cannot kill anymore, as Black can set up an oshitsubushi as shown.