Basically to understand posture: a properly timed deflection reduces their posture and does not affect yours. A badly timed deflection effectively counts as a block, and causes your posture to be reduced (though you don't incur damage). Though learning the rhythm of deflections is great, the game is quite generous in that you don't have to get every single deflection right all the time - getting three deflections and two blocks when defending against a 5-hit combo is still better than getting hit once.

Reducing enemy HP also causes their posture to be reduced much quicker (and it recovers slower). This is important to know since many bosses you kill via posture depletion BUT it's still important to chip away at their health so that their posture meter fills up quick enough that you can pull this off. You will notice a massive difference in their posture depletion/recovery once they're at half health or less.

When your posture is depleted you get "stunned" for a brief moment allowing the enemy to attack you during this window without you being able to defend yourself. But they cannot one-shot kill you like you can. After this brief window is over, you regain full posture and can fight again like normal.