If you Block with a Sword/Shield after partially breaking a Block, the block will keep partially broken. You can finish breaking it if you don't release Block, and it will keep it's partially broken timer. Shields need a bit more precision when doing it, as you can't attack and block at the same time. Also with Shields, you can release Block, and the block will stay partially broken, but if you attempt to break it again, the Timer will reset.