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[kmonster]

In the gold box games you can only backstab a target if you attack opposite to the first attack at the target in that round and the target didn't act in the meantime.

When you attacked a target and the game displays "attacks ... from behind" you were in backstab position. Non-thieves don't get a backstab multiplier, but they still get the bonus to hit.
Backstabbing isn't really needed versus hobgoblins or other low level monsters which die from a normal hit, but it's the best way to beat high level enemies in physical combat. In DkoK my enlarged thief could backstab the end boss for over 100 damage with a single attack.