What rules govern this kind of decisions?

You've got a couple of goals here.

(1) Bear off quickly. In some cases you may volunteer a shot to bear off more quickly.

(2) Try to bear off in such a way that you keep a strong board as long as possible,

(3) When you have to open a point, ask yourself if getting hit costs you anything.

In this position 6/off wins on all counts. Black gets a checker off, gets a 5-point board which is likely to last at least one more turn, and is actually better off getting hit if White enters.

Thank you I had actually never considered why taking off the 6 is so much better than taking off the 5 (and leaving a blot of course). Trying to get the board closed as long as possible certainly makes sense

But why is 5/4 5/off so bad, here?
white has 15 crossovers before the roll to black 12. that is 3 crossovers difference
I remember one of your rule saying that:
-2 or less crossovers difference play safe
-5 or more play bold
-3 or 4 is the grey area where you take into account other criteria such has:
-3 tend to play safe and 4 tend to play bold
-have I an odd number of checkers born off after the play : yes
-does opp have a blot in his board: no
-does opp have a bad bearing off structure: no
-do I have a speed board: no
I understand this is not an exact science but here 3 of the criteria point to safe and the safe play ends up being a big blunder (0.130)

What feature of the position makes it so compelling to leave the blot when I am not forced to?

Here is a constrasting position where all main contenders are tied and there is only a 1 crossover difference.
I would have expected the decision to be not even close.

I am venturing a guess that the additional chances gained by hitting him again with the recycled blot make it worth leaving said blot exposed. I wouldn't have thought so at first glance though.

I learned his "rules" for bearing off against a checker on the bar with a closed board different. With a closed board:

a) When you lead by 5 or more crossovers make the super safe play;
b) When you are in the range of leading by 4 crossovers to trailing by 2 make the play that combines speed with safety; and
c) When you trail by 3 or more crossovers, make the speedy play.

(My coach John O'Hagan quoting Bill Robertie's rules)

6/off is the speedy play that will keep your board stronger for longer. Obviously 5/off 1/off is the speediest play, but it is too much for here. 6/off in comparison to 5/off 5/4 allows to play better in following rolls when your opponent rolls one of the 25 rolls that keep him on the bar. There are more rolls allowing you to take at least one off and keeping a 5 point board after 6/off, where youa re more likely to have to break another point after "safe play."

good posts guys. thanks for the great tips!