This article is from the Backgammon FAQ, by Mark Damish email@example.com with numerous contributions by others.
From the FIBS help screens:
If you are playing an n-point match and your opponent is ahead
of you and he gets to n-1 points you are not allowed to use
the doubling cube in the next game to come
EXAMPLE: 5 point match score game # You opponent 1 0 3 2 0 4 3 1 4 (you were not allowed to double in this game) 4 3 4 (you were allowed to double again) ... ... ...