A successful Mind Control attack by an alien immediately converts the soldier to the alien's side for the duration of the turn and gives them a full complement of TUs to do actions with - such as shooting teammates in the back. The soldier will continue to be on the alien's side for your next turn, will use reaction fire on your own troops, and will even be represented on the map as an alien red dot when spotted. At the start of the alien's next turn, the soldier will revert back to your side (unless the enemy has performed yet another successful Mind Control attack). The same applies in reverse when you control an alien.
Wary commanders recommended dropping weapons and grenades if you fear your men may become mind-controlled (or even panicked to the point of becoming berserk - berserked soldiers shoot the nearest individual, which is almost always your own team). Mind controlled troops will even take weapons out of their backpack or reload them. Anyone who has been successfully mind controlled should be marked as psi poor (rename them on the Soldiers screen), and fired or used only for non-psi missions (no Ethereals or high-ranking Sectoids). For the same reason, you should not be too queasy about shooting any of your soldiers which are under alien control; you will probably have to sack them anyway for being psi-weaklings.
Psionic control is so taxing, it only lasts for a limited period of time (that turn). It has to be re-established every turn. During this round, control is absolute, and even outright killing of the controller isn't going to snap the victim out of it, much less mere distractions. Even if no orders are given, and the controlling unit dies, the victim will still obligingly take potshots (reaction fire) at former team mates.
Note that aliens under mind control are considered 'friendlies' and do not give you credit or experience if you shoot them. Also, if one of your soldiers is under alien control when the last alien is shot to end the mission, that soldier will become MIA.
Mind controlling 4-square units - a bug
The alien robotic ground support weapons (Cyberdiscs and Sectopods) have psi receiver circuits which allow them to be directed psionically by their squads, much as our own HWPs use a radio link. However, these units are even more difficult to control than Ethereals. Furthermore, studies have shown that all "large" alien species in fact have a brain/ control module implanted in each of their four quarters. Each successful mind control attempt will only affect one quarter, leading to... strange things. Such as being able to attack, or be attacked by, the other quarters. Or the other three quarters take the fourth for a ride - and the creature basically attacks you as usual. The North West Corner quarter seems to be the most crucial piece, similiar to our HWPs.
Mind control and stunning
- If you stun your soldier while they are under alien mind control, they will be on your side when they recover. This is an effective way to save lives.
- The flag that remembers which side a unit was on before they were mind controlled has some bugs. The most obvious is that a mind controlled Civilian will always revert to Hostile - so arming them using the inventory screen exploit will not result in free support fire, but rather additional enemies.
- A less obvious bug is that a zombie under your mind control, when killed, will hatch a hostile Chrysallid that turn - who will revert to your control at the end of the turn. See Gaining permanent control of a Chryssalid/Tentaculat.
--JellyfishGreen 06:11, 26 Aug 2005 (PDT) & notes from Jasonred
Mind control and reaction fire
Alien units will take reaction shots at other aliens that you have mind-controlled, but your soldiers will never take reaction shots at a fellow soldier who has been controlled by aliens. If one of your units is armed and mind-controlled, kill or stun them quickly on your turn. If they are fully disarmed (this includes all grenades!), they are harmless, and furthermore your other units will not attack them.
Mind control and alien artefacts
Even if you have not researched a weapon an alien is carrying itself can use it when under your control. This comes in quite handy if you want to destroy that outer UFO wall but couldn't fit in the blaster launcher into research schedule.