Dark Sun Campaign Setting Revised[]
Though spellcasters can mumble verbal components and hide material components in their hands or robes, somatic components are harder to hide. The somatic component of any spell, magical or clerical, is apparent to any character watching the spellcaster. On Athas, where spellcasting is often illegal, the ability to hide the necessary gestures becomes important. If movements can be concealed, a spell can be unleashed without calling attention to the caster.
A character using the somatic concealment proficiency must announce to the DM his intention to do so at the beginning of the round. Then, when the character casts his spell, the DM makes the check in secret. A successful check indicates that anyone who can see the wizard doesn't recognize his gestures as being magical in nature. A failed check means that all who can view the casting wizard see his movements for what they really are.