Shadow-Skinned: The Mandrake has special chameleon like qualities that make it possible for them to blend into the background, even where in open areas.  Mandrakes always count as being in cover, gving the a save of 5+ against most attacks.  This counts as assault as well, so...unless the attacker is armed with nasty frag grenades the Mandrakes have first strike.  If attacked by something like a Lictor (Tyranid) that ignores the cover then the normal 5+ save applies.

Hidden Deployment: Mandrakes are deployed in a sort of "abnormal" way.  Mandrakes ALWAYS deploy at the start of the mission no matter what the missions special rules say.  However...instead of placing the unit on the table as you would a normal unit, take three models (no matter how many are in the unit) and deploy them seperately any place the missions rules will allow.  Any ONE of these models can represent the actual unit, though you don't have to decide which until it is time to for the squad to be revealed. 

Until the squad's location in revealed no one may shoot, assault, or otherwise attack the model's.  This rule goes both ways though.  The Mandrakes models cannot affect the game.  They can't be used for crossfire purposes, to block line of site etc.  All these individul models can do is move up to 6 inches.  Since they can't attack there is no attack phase for them so they can't use the fleet of foot rule either. 

You are allowed to reveal their true location at the begining of any phase of your turn.  BUT you MUST reveal it by the end of your third turn.  After they are revealed take the two fake models off the table and deploy the rest of the unit withing four inches of the remaining model.  The Mandrakes cannot return to their hidden status for the rest of  the game.