Each Boss acts differently.
Chase Down: Damages nearest unit for 4. Swing Rush:Damages farthest unit for 3. Mass Constrict: Moves adjacent to most possible units at once and rolls 2 black Attack Dice (deal result to all units).
If a Beast is combat-locked when initiating a special attack (rolling 6) he disengages from the combat-locked situation and is repositioned following the special attack's instructions, becoming combat-locked again(it's possible that this new position is the same hex).