Psychological Tesuji / Black keeps sente

False Tesuji  

seems to be a tesuji, but in fact it is not.

Gobble, gobble  

If White responds to with the seemingly natural move of , Black would keep sente because puts the White group into atari and forces .

Possible refutation  

Instead, White can play as shown.

Possible refutation, continued ( at )  

After , Black could capture back, but that would lose sente.

Variation ( = tenuki)  

To retain sente, Black could play elsewhere instead, which allows White to follow up with the sente sequence of -. In terms of points, the resulting position is equivalent to the alternative of allowing White to play at in sente earlier (Black captured one White stone, while White has captured two Black stones and has an extra stone at ). But this position is worse for Black because the ko threat at a is lost (see aji keshi). If Black wanted to take sente, it would be better to simply play elsewhere rather than playing out the entire sequence from to , which loses one ko threat for nothing.

(Note: Technically, in the third diagram is wasting a ko threat as well. If Black is not going to follow up locally with , it would be better to play elsewhere as early as . But even then, White can still play at , which again forces at and leads to the exact same final board position. Besides, this result is still inferior to the alternative of playing elsewhere with from the very beginning, as is a smaller ko threat than a in the scenario where is absent.)

Conclusion  

With all of the above in mind, the trick play at a is clearly worse than the correct play of .