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Keywords are:  'thief skill contortionism'

Skill: contortionism            Lvl 23 Thie  (42%)
                                (important)   Requires: dodge
                                Group: Thief defensive fighting skills

This is a passive skill, used by thieves to get into tight locations.  By
learning this skill, a thief will be able to squeeze into and through much
smaller places, often allowing them access to entryways not available to
other characters.

This skill is used automatically when walking.