British Go Journal No. 20.  July 1973. Page 11b.
Some strange things have been happening at my 2-1 points recently.When Black has already supported a corner start stone with an ikken tobi , white 1 and 3 in Dia 1 can be used to live in the corner. After black 8, black threatens to start a ko for the life of the White's group at any time with A or C, white replying at B or D respectively. I have been caught napping in this situation twice recently.
Diagram 1 ||
Diagram 2 |
Of course, white can make unconditional life by playing B or D after black 8, but as this is gote, and the white corner could still be completely cut off from the outside and reduced to four points by black in sente (see Dia 2), White hasn't the time to do this in a handicap game.
Diagram 3 ||
Diagram 4 |
14 at 12.
In a recent game, when I guessed Black was plucking up courage to start the ko, I played A in Dia 3 (instead of B or D in Dia 1) hoping to make an extra point and good yose along the edge as well as unconditional life. What a mistake! Black exchanged his 1 for white 2 and then played at the 2-1 point, threatening 4. Black 5 threatened to connect wih his 1 killing the group and damezumari  prevented me from separating them at once, so I still had a ko fight starting with white 6. At least Black had to find the first threat.
A few weeks later I was in the same position, Dia 4, and was able to play the hane tsugi  1 and 3 in sente. Pleased to have avoided the ko in sente, I played white 5 elsewhere, oblivious to the appalling weakness at my 2-1 point. Black 6 threatened three stones with a snapback, and after the forced sequence to 13, recaptured at 12 with his 14 to leave a formation that reduced to bent four in the corner. I had indeed avoided ko with sente!