In the evaluation settings dialog box you can specify whether or not checker play should be evaluated cubeful. It's recommended that you use cubeful evaluation. To get an understanding of what cubeful checker play evaluations are, you can take a look at this position:
In this position black has rolled 51 and he has a good position. If the position is evaluated cubeless the best move is 13/7. Black can hope white does not roll 34 or 35 form the bar and has now a good chance to close white out in the next few rolls. However if white rolls one of the four hitting numbers from the bar, white will quite soon have a really hot redouble. This redouble increases whites equity so much that black actually should play this move safe. He should play 13/8 6/5. However if the evaluation was set to cubeless, an setting which assumes white will never redouble, black should play 13/8.
Here's another example:
Cubeful example 2
This is from a 5 point match where black has 1 point and white has 3 points. Black wins the opening roll and considers playing 13/11 6/5 or 24/23 13/11.
If GNU Backgammon uses a cubeless evaluation it will play 24/23 13/11. But if you're using a cubeful evaluation it will play 13/11 6/5. Slotting with 6/5 is at this score a better move even though it loses more gammons. The gammons black are losing won't matter anyway, since black will turn the cube in the next few rolls anyway. The slotting play also wins more gammons, and with the cube tuned to 2, black should play towards gammonish positions at this score.
You can read more about cubeful evaluations in the [[Appendix]].
It's recommended that you use cubeful checker evaluations.