Bulky seven in the corner
If White plays first, this shape becomes a seki or a 10,000 year ko. Unless White is komaster, Black will normally play elsewhere, as White only threatens a 10,000 year ko, and the best Black can do is seki, while in the original position Black could live with 6 pts. of territory.
(If Black has 2 or more outside liberties, White cannot prevent Black from capturing the two white stones.)
(Note that if black chooses to play elsewhere and accept a possible 2-step ko, he must do so after 3. If black tenukis after 1, white kills with 2 then a3. kevinwm)
Both players would prefer that the other fill a first (White must take before filling a). Since White has to play here four extra times (, , captures , and finally fills in a) and have an extra ko threat to win this, it will be very hard for White to find time to start this, but it might be possible depending on the rest of the board. The rest of the game (after white plays ) might become a fight to get an extra ko threat!
Black could also make seki this way. But this is worse technique, as is absolutely necessary; Black does not have the choice to omit .
threatens to make a two stage ko at a instead of a 10,000 year ko. If White captures the Black group, it will take one less play than the 10,000 year ko. However, if White is komaster Black will make seki at a, anyway. is technically inferior because it allows if Black is komaster.