Kitani took a sente as his sealed move, and Shusai was enraged when play resumed and the move was unsealed. Traditionally, the higher ranked player had made the last play before an adjournment.
In the 1970s I read an interview of Kitani in a Japanese go magazine. When asked about the move, Kitani replied, "Well, he answered it," and chuckled.
Now, it may well have been the best move at that point. But I doubt that that was why it was played. As a double sente, what the move gains is infinitesimal. (See http://senseis.xmp.net/?GoInfinitesimals ) Instead of leaving a Black sente against the White group, it leaves an ambiguous play (See http://senseis.xmp.net/?AmbiguousPosition ), such that White does not have to answer a Black descent. White gains a tempo in any possible contest to get the last 1 point play. (In the actual game, Black could have gotten the last 1 point play, but let White get it anyway. Doing so did not cost anything in the end.)
Except possibly for thinking time, I do not believe that Kitani's ploy made a difference in the score. In the SGF file below I have assumed that the game would have continued as in the actual game, except that White gets the sente against the corner before Black plays the sente against the White group. Then I play out the game from the start of the 1 point plays as the players did, except for the difference in the sente, and Black gets the last 1 point play, as he should. (Letting White get it would not have affected the score.) The end result is the same.
