There are 2 nationally known and world-travelled dommes who have at times posted at max and who use the phrase breaking a sub in similar ways, though their meaning is maybe not identical.

For both of them it seems that breaking a sub includes but is not limited to the prisoner interrogation scene. It seems to include when the guy starts crying, begging or changing his limits, but obviously, no two dommes are identical!

For them, a guy giving up a secret word or idea is one form of breaking, even if it is not emotionally traumatic. At least, one of the mistresses has used the words break or breaking to describe a prisoner interrogation scene!