Here is my attempt at the Nactation: 62S-41X-21H-43H-41H-51.
I would have played B/20 6/5* quickly and I was surprised when XG liked B/20 22/21. Here are three reasons that hitting (B/20 6/5*) is weaker than B/20 22/21.
(1) Hitting leaves 26 return shots (all but 33 66 64 63 62 61). When you hit loose on your 5pt on the second roll with 54S-21, you leave only 21 return shots. When you are leaving 26 instead of 21 shots, you should make sure you are getting enough compensation on the 10 misses. As we see in (2), Blue is not.
(2) One objective of hitting is to reduce the number of point-making rolls your opponent has from the roof. As Casper pointed out, White's misses all (except 66, which fans) can be used to play 17/11. After Blue hits, all 35 of White's entering numbers either hit or make a point. In this position, hitting is not effective in giving White really bad rolls from the bar, except 66.
(3) If Blue hits and White misses, Blue is short on covers. Some of the non-covers are sick rolls.
Take a look at the probability distribution shown in the rollout result. Blue wins more games after not hitting. Blue wins more gammons after hitting (13.0 – 12.4 = .6%) but he also loses more gammons after hitting (10.8 –9.5 = 1.3%). Note that Blue has a net gammon loss by hitting. If you think you cannot lose a gammon while owning the 20pt, then think again. If you hold the 20pt until the end of the game, you are likely to lose a backgammon.
Now that you know the money play, see if you can figure out the Gammon Go play.
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eXtreme Gammon Version: 2.10