Where you block 100% of the damage and it drains from your stamina...
However if the attack is too strong it drains all of your stamina and some hp.
It makes sense because in real life if youre strong enough to block an attack youre not going to get hurt...
However if someone, say a jacked up bodybuilder swings a warhammer at you and youre a scrawny weakling youre not going to block it completely and will probaby break your wrist, arm or tear some tendons and ligaments.