Some spells like the Gold Magic spell The Gilded Cage and the Amethyst Magic spell The Manacle of Caloe trap models and prevent them moving. Other spells such as the Celestial Magic spell Crystal Charioteer and the Light Magic spell Abulla's Snare take models and move them around. As a result models may be separated from their units.
Obviously, for characters and monsters which can move independently this isn't a problem, but what about rank and file troopers and Champions who must stay with their units? If a unit is divided as a result of a spell then the two parts may be treated as two separate units until both parts are free of the spell that split them up. A part might be just a single model, or any proportion of the unit.
Once free of the spell, the smaller part must attempt to rejoin the larger part as soon as possible. The smaller part may not declare any charges while trying to rejoin the main body and it will not pursue enemies broken in hand to hand combat. The larger part of the unit may fight as normal. As soon as the two parts touch, the models in the smaller part are rearranged into the ranks of the larger - doubtless heaving a sigh of relief at being reunited with their comrades!