Hammer Block is an Active Physical Skill that allows Scarlett to enter a defensive stance with any War Hammer or Battle Axe weapon to Block other Hammer or Sword 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 possible attempt at performing a Combination and leaving them open for a counter stroke. (To perform a counterattack, Scarlett can simply drop the blocking stance immediately after blocking the enemy's strike and attack.)
Hammer Block has only a single Level of Mastery. Scarlett can only invoke this skill when she is wielding a War Hammer or Battle Axe weapon (there are separate Blocking skills she can learn for each of the other Weapon Classes.) Hammer Block is only effective against Hammers, Swords and Sythe Weapons (such as the Darkside Moonblade). It is not effective against Spears.
Hammer 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 the flank or rear.)