Poise
Poise is how much damage a creature can take in a short time before its stance breaks and it staggers, leaving it open.
Also called stagger, stance break, poise damage.
Big enemies that flinch at every poke stop being threatening, and big enemies that never give way get tiring to fight. Poise sits between the two: the creature keeps going through your hits until enough damage lands in a short enough time, then it staggers.
How does a stagger work?
The creature has a poise value. Damage fills it up, and a pause in the damage lets it reset. When enough damage lands quickly, its current attack is cancelled and it stands still for a moment. Making it take extra damage while staggered turns that moment into a reward for a well-played sequence.
Poise in Mytheria
The heaviest creatures, marked steadfast, never flinch. The Stoneback Colossus has 90 poise: deal 90 damage without a pause of more than four seconds between blows and it staggers, cancelling its attack and standing still for two and a half seconds. It normally takes only half damage from your blows, and while staggered it takes double. Hitting it in the last moment before an attack that's warned in blue staggers it too.
The full post, with clips and demos How to make big enemies feel heavy in combat