Sword Block is an Active Physical Skill that allows Scarlett to enter a defensive stance with any Sword to Block other 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. There is also an explicit Riposte skill available.)
Sword Block has only a single Level of Mastery. Scarlett can only invoke this skill when she is wielding a Sword (there are separate Blocking skills she can learn for each of the other Weapon Classes.) Sword Block is only effective against Swords and other Sythe Weapons (such as the Darkside Moonblade). It is not effective against Hammers or Spears.
Sword 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.)
Scarlett can learn this Physical Skill from any of several of the Physical Skill Teachers she meets: Brawler, Yngvar, Rangar, Ulrik, Abd el Malik and Akbah. Scarlett can also practice this combat technique with Rangar as a Sparring Partner.