Introduction
The lesson video is the following
Main position
In this lesson we have seen how to defend the corner in the following situation.
Usually, black will choose C to get the largest corner possible. Unless white is extremely strong outside in which case A and B are playable.
When black choose C, the following sequence is played.
If white as a very big move elsewhere, he might consider to tenuki with A. But there is still aji in the corner so to perfectly secure the corner, black should defend with either B or C.
Black tenuki
If black tenuki, the following sequence should be played.
It is important to note that black has to give away part of the corner by playing 8. If black is greedy and try to keep all by blocking down at 8 as shown below.
Then white will play the following sequences. With black 14, white is locally dead but white can cut at 15 and start a fight. It is hard to play for black as he must protect two weak groups.
Black protect at B
When black protect at 4, as shown in the next diagram, the invasion at A becomes impossible.
Indeed, if white tries to come in, the same sequence as previously is played, But now, the stone at 4 is nicely place to create a tiger mouth making it impossible for white to play the cut.
Black protect at C
The same idea is behind the move at 4 in the next diagram.
Again, if white tries to invade with 7, the same sequence is played, but the cut at 23 doesn’t work as black can simply play at 24.