Both you and the enemy have their own posture gauges. When blocking enemy hits, you take posture damage. If an enemy blocks your attack, you deal damage to their posture gauge. Once the posture gauge has been fully damage, a posture break will occur, leaving the enemy open for a deathblow. This can occur even when the opponent still has some HP left. This is why posture break is an important combat mechanic of the game.
Powerful attacks can easily damage the posture gauge. For Sekiro, his charge attack/lunge can break normal enemies’ posture. Blocking powerful strikes from enemies can heavily damage Sekiro’s posture gauge. If his posture breaks, he’ll be stunned for a few seconds, leaving him open for enemy attacks. Don’t worry since his default posture gauge is sufficient enough to protect him from a flurry of attacks. You can further increase your posture gauge (and max HP) by collecting four Prayer Beads.
You can protect your posture gauge by avoiding attacks and by performing deflections and counters. Skills like the Mikiri Counter can heavily damage an opponent’s posture gauge if you pull it off successfully. You just have to be careful of unblockable attacks since these will damage you directly.
Your posture automatically recovers as long as you’re not blocking attacks. You can recover even more posture by guarding (holding the L1 button) without taking damage. To do this, you have to guard while moving around and keeping an eye on the enemy. Vitality and Posture works hand in hand. The lower your vitality is, the slower your posture recovers. The posture gauge’s color will get redder as the recovery rate slows down.
It’s recommended to use Dodge Counterattack (press Circle to dodge then R1 after dodging) against enemies with fast Posture recovery. There are enemies that can’t be defeated even after you deplete their HP bar; these particular enemies require a complete posture break so you can finish them off with a deathblow.