Help me in understanding this

[Cafeplayer]

See the situation below (GnuBG Position ID: tfYFBgC2dRYYAA Match ID: cAlvAAAAAAAE).

White has rolled 62. The safe play obviously is 9/3 8/5 but since red has three points on the board slotted one might be tempted to leave a shot open in order to gain a slightly better distribution for the bear off and to provoke a hit. After the move white is still 1 pip behind and, of course, red is rolling thus some contact wouldn't be bad.

So the first question is whether it actually is better to leave the anchor or to play safely and hope for a bit of luck in the next rolls. I would go for the risky play because red's board is not too badly positioned for bear-off (making it less likely for white to win the race) and the risk of losing a gammon even when hit is not so big.

Now I thought after moving 15/9 with the 6 it would be sensible to cover the 5 point with 8/5. Gnu thinks this is worse by 0.047 than the move 15/9/6 which it rates as best move overall. Why is it better to leave the 5 point open in this situation?

[dorbel]

If Gnu likes 15/6 best then you probably have some settings wrong somewhere, as that play is clearly inferior to 15/9, 8/5. You can see this quite easily if you ask yourself, "Will Red hit next turn?". If you make the 5pt, she will only hit with a 3-3, but if you leave the 5pt slotted she can also whack you with 1-1, 3-1 and 3-2.
The really interesting question is, should White be running off the anchor at all? I think the answer to that is no. "When you're ahead, race, when you're behind, stay back" is a good guide, but there is a sort of grey area where, although behind in the race, your chances can be better in a pure race than they are if you try to maintain contact. That area tends to be in the region of 40% chances in the race with the opponent on roll, so it's well worth considering here, but a rollout (1296 games on XG2) demonstrates that is is slightly inferior. This is probably because the straggler doesn't always get safe next turn, failing to get into his outfield with 3-3, 2-1 and 1-1, an 11% chance of course. Running is a very small mistake, even with all those blots in Red's board.
An interesting corollary is that if you had rolled 5-4, then running is definitely wrong! This implies that leaving one man back acts as a sort of pseudo anchor, forcing Red to play most of her rolls without playing from her midpoint which is what she wants to do next!

[Cafeplayer]

Thanks, dorbel, for the reply. As you can see below GnuBG insists on 15/6 being the superior move even after a roll-out. Maybe because it makes red more willing to hit the lone remaining checker on the 15? The probability for losing (as well as winning) with a gammon is higher than for the other options.

Whatever, probably not worth spending more time on this problem.