TwoSix
Everyone's literal second-favorite poster
Where did I say a creature would always act in the same way? I am asking how any and all creatures are auto-magically aware of this maneuver and it's resulting effect.
Because you're failing to distinguish between the monster as fiction device and the monster as game construct.
The monster as game construct knows the effect. The DM takes into account the effects on the monster, the monster's abilities, and other mechanical considerations, as well as fictional constraints on the monster (like "I'm a mindless ooze").
The DM than has the monster take an action which will provide the most payoff, where the payoff rewards can vary between satisfying the desire for challenging combat and providing fictional accuracy (and hopefully provide both!).