XGID=-abBcBBDBA---A-aAc-c-a-a--:1:-1:1:21:0:0:3:0:10
Unlimited Games. How should Red play 21?
This is the type of position that was misplayed for years before the advent of the bots. The most common play would have been 7/6, 7/5, leaving blots so that Red could correct his timing.
The bots have taught us to make a more draconian approach. If Red makes his 9-pt to build a five-point prime, then he doesn’t need his 3-pt and those checkers can be recirculated, and Red’s timing can be improved.
It is difficult to find 16/15*, 3/1* without having seen the technique before and even then, it looks frightening. You can imagine the arguments that would ensue in a chouette over the right play here.
I have no reason to doubt XG’s play here. In fact, it rates any play other then 16/15*, 3/1* as a blunder. I recommend playing out the position a few times to get an understanding of the technique involved. The point of 16/15* is to give Red a six next time to cover his blot on the 9-pt.