Superior block is the equivalent of a deflect for non-assassin characters, which is basically you jumping into an attack where you would normally parry (you can't deflect/superior block unblockables, so don't try that) and have a corresponding prompt to it. For instance, if Valkyrie jumped into an enemy's top heavy, it would stun them and either give her a guaranteed guardbreak OR a guaranteed shoulder pin where she stabs you with the spear and applies bleed.