This spell appears to be a high-level body-control technique that allows the user to control a target body to follow a set of instructions of whatever the caster wills. Professor Quirrell used it to make Snape return to guarding a door but without announcing Quirrell's presence. His unconscious body would give basic sentences about staying away from the corridor to people who walked by. It is unknown if the target must be unconscious for the spell to properly function but being an occlumens apparently does not protect one from the effects of the spell.
"Professor Quirrell then turned back to where the Potions Master lay sprawled, bent over and placed his wand on Professor Snape's forehead. 'Alienis nervus mobile lignum.' The Defense Professor stepped back, and began to move his left fingers in the air as though manipulating a puppet on strings. Professor Snape pushed himself up from the ground by smooth motions, and stood once more before the corridor door."