I found it interesting to compare this 53P-61P-41 with a 2nd roll 61P-41 (as rolled out by Stick).
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For the second-roll position 61P-41, Split is best. It is a common theme to split the back men after the opponent has stripped the eight point to make a point. It improves the chance to make an anchor, and threatens a direct shot at the eight point should the opponent break it to make an inside point. Meanwhile, bringing a man to the nine point is a powerful builder for the nine point itself and deeper points.
So why should there be a swing of 0.032 in favour of Up (24/20 24/23) if Blue has already made the three point?
A few factors spring to mind:
- Blue is 8 pips better off in the race. This encourages him to advance a man to where he can leap out with a large roll.
- Blue's improved inner board is more threatening should White hit loose on his 5 point.
- An initial 41 allows Blue to make the 3 point with 63 65. Those rolls are not so good once the three point is already made; but if he has advanced to the 20 point they can be used to bring that chequer to relative safety in Blue's outer board.
- Blue's own eight point is stripped. This reduces the value of a later 43 54; although they make a valuable inner board point, this now comes at the cost of leaving the eight point blotted.
Having seen this result, I expected U to be best for 42P-61P-41 and 31P-61-41. However gnubg evaluation has 42P-61P-41 [S D36] \e and 31P-61P-41 [S D25] \e. Perhaps the more compact priming formation makes slotting the nine point advantageous once more. If you have rollouts for these two variations, I'd be interested to see them.