is the vital point. It clearly makes one eye at the circled point, and White can't prevent another eye to the left of .
Once the vital point has been played - in the main line - the rest is not so difficult any more. Finding that point is the main challenge here. Let's have a look at how things can go wrong.
One is tempted to cover the two-space gap with a move like here. However, White strikes at the vital point. If at , then at .
This results in ko.
ChrisR: White can avoid the ko by playing first, then . Black's group is dead.
Yohei: Black is not dead - he can make seki as shown below.
ChrisR: Playing at is a failing move, but it is better than playing at 3. Up to , this is seki.
Yohei: White won't be so kind. in the following diagram kills.
Go to Problem 41