Shield Block is an Active Physical Skill that allows Scarlett to enter a defensive stance with any Spear and Shield weapon to Block other attacks of her enemies. Successfully Blocking an Attack prevents that attack from causing any damage to Scarlett's Health.
Also, successfully Blocking an attack will momentarily throw the attacker off-balance, disrupting any Combination and leaving them briefly open for a counter attack. (There is also an explicit Covert Strike skill. q.v.)
Shield Block has only a single Level of Mastery. Scarlett can only invoke this skill when she is wielding a Spear & Shield. Shield Block is effective against other Spears as well as against Swords and Sythe Weapons (Darkside Moonblade). It is not effective against Warhammers. Scarlett can only block attacks from enemies in front of her.
Shield Block is an Active Skill; it needs to be placed on the QuickBar so it can be used. Once Scarlett enters the defensive stance, she can hold it for as long as the designated key/button is pressed. Using the skill does not cost any Mental Energy.
Enemies may attempt to circumvent Scarlett's Blocking Stance by maneuvering to flank her; Scarlett can only block attacks from enemies in front of her. (Likewise, when enemies block Scarlett's attacks, she can Maneuver around them to attack from their flank or rear.)