For this week's haengma, I decided that I would cover the invasion of the joseki discussed within this week's joseki post, so let's get into it!
Please understand for all my posts, that I am nowhere near the level of a professional's and my posts on this forum are just to keep things lively. If I make any mistakes within my posts, please let me know!
So when playing this joseki(within the diagram below), you often see that black will invade at A, to destroy white's territory on the side.
However, a common mistake that I find when seeing this invasion played, is that black's response to white's covering move is often wrong! So for this invasion, it is proper to play at A and not B, and I will explain down below.
Playing at B is often tempting for players as it seems to be the weaker side, but upon inspection, we find that this is not the most effective way of invading! So the diagram below is an example of black's attempted invasion on the lower side(this may not be the best variant). Although it may look like black got what he wanted from playing at 7, we see the thickness that black had helped white build and the newfound weakness within the upper side as white became strengthened. So this result is not good for black as he settles by crawling on the second line.
(The squared stone represents move 7, and the triangle stone represents move 10) With the variation of the cut, at 9, which seems promising at first, we find that black will struggle as the ladder doesn't quite work because of white's stone(triangle). So we find this invasion to be a failure for black as well, as he misses the opportunity to invade and only provides more strength to white.
So now let us study the variation played at A.
In this variation, if white chooses to cut at 8, black gets the opportunity to ladder the white stone if white fails to respond.
If white does choose to save the stone, then we see the overall result is still not good for white if black gets the ladder on the triangle white stone within the lower section. So white cannot cut at 8 unless the ladder does not work for black on either sides.(Please note that this also means black should not play this invasion unless he has the ladder on the lower side)
So in the case that both ladder works for black, white cannot play the cut at 8 and must cover at 8 in the diagram below instead. The diagram below is a representation of a possible outcome of white covering black. In this variation, we see that although white is strengthened with influence towards the middle, black also settles well, so this invasion is playable for black.(Once again, this may not be the best variant) In a different variation, although there is no diagram for it, white may think to cover at 11 instead of 10, and invade or reduce the corner later.
I understand that I didn't cover all of this invasion variants and methods of countering, but I hope this was at least helpful to beginners learning the game! Once again, I am only an AGA 3D, so I may mistakes and so please add on or correct me when you see something wrong! Thank you