The mechanics of how thief abilities actually work in-game is left somewhat vague in the rulebooks. This page provides a gameable system.

DMs should keep in mind that if they make thief abilities bad, or a pain to use, thieves will suck pretty bad in their campaign. Thieves are not good at combat and can't cast spells. Don't gimp their cool abilities; they won't be overpowered.

Whenever possible, the player should roll the thief ability check. The only time this is impossible is for Find Traps (FT). Even for Hide In Shadows (HIS) and Move Silently (MS), oftentimes the DM can put the dice in the player's hands by postponing the roll until the moment of consequence.

Success means the thief is no longer a valid melee target. On a subsequent round, the thief can break HIS to obtain a rear attack + BS attempt on a target within 1”. The thief could also prepare an action to BS anyone coming within 1”.

Success on HIS also gives +1 to surprise if the thief is 90’ or more away from any moving/noisy allies, but this does not stack with MS, since they can't be used in conjunction (HIS is stationary, MS involves movement).

This is done in conjunction with a close-to-striking-range action (action type E in the combat procedure). Success means the thief obtains a rear attack + BS attempt the following round against a target within melee range that is otherwise occupied (e.g. looking elsewhere, fighting other characters in melee). The MS dice should not be rolled until the following round, when the BS is to be attempted, in order to keep the player in the dark.

Success on MS also gives +1 to surprise if the thief is 90’ or more away from less-silent allies, but this does not stack with HIS, since they can't be used in conjunction (HIS is stationary, MS involves movement).

A BS attempt can be obtained without HIS or MS if the thief gets a rear attack by some other means.

BS while dual-wielding grants BS with both weapons. When initiative is tied and the thief gets multiple attacks due to speed factor, if the thief has a BS attempt, all attacks are BS attempts.

Multiple attacks in a single routine all qualify for BS. In the case of multiple routines, only the first routine qualifies for BS unless attacking from the rear.

In the case of multi-class characters, backstabs can be performed with any sword, dagger, or club, even if it is not a thief weapon.

Thieves can wear armor other than leather, but are subject to penalties as per UA 72 (or UA 22 pre-errata). Make sure to read all the footnotes to that table.

Magic armor (other than leather) reduces the penalties by 2% per plus of the armor.^Ex

Monks also obtain the listed bonuses for wearing no armor.