During the Declare Battle Formations step, if every model in a unit has this ability, you can set it up in Reserves instead of setting it up on the battlefield. If you do, in the Reinforcements step of one of your Movement phases you can set up this unit anywhere on the battlefield that is more than 9" horizontally away from all enemy models.
While an enemy unit is within Engagement Range of this unit, halve the Objective Control characteristic of models in that enemy unit.