The creature can’t take actions voluntarily. Instead, the dominator chooses a single action for the creature to take on the creature’s turn: a standard, a move, a minor, or a free action. The only powers and other game features that the dominator can make the creature use are ones that can be used at will, such as at-will powers. For example, anything that is limited to being used only once per encounter or once per day does not qualify.
The creature grants combat advantage.
The creature can’t flank.
In spite of this condition, the creature’s allies remain its allies, and its enemies remain its enemies. If the dominator tries to force the creature to throw itself into a pit or to move into some other form of hindering terrain, the creature gets a saving throw to resist entering the terrain.
A wide variety of creatures and powers can impose this condition. Just as in myth and legend, vampires and fey creatures are adept at controlling the minds of others, but the specific limitations of this condition prevent such creatures from forcing player characters to expend their best powers.
Published in Player's Handbook, page(s) 277, Player's Handbook 3, page(s) 220, Rules Compendium, page(s) 230, Monster Vault: Threats to the Nentir Vale, page(s) 123.