How to give players in combat hint of a hidden boss mechanic?

I'm making a boss for a one shot, I have a basic idea for a mechanic where if they can manage to disable the boss's mouth, it will greatly limit its abilities. Pretty much all of the boss's abilities will be accompanied with me describing how it roars/howls out the spell, but how do I communicate to my players that this is more than just descriptive flourishes? Should I just balance the boss assuming they won't figure this out and have this be a reward for creativity?