All playable characters in Hyrule Warriors can activate Focus Spirit when their Magic Gauge is full.
While Focus Spirit is activate, attack power and speed increase, incoming knock-back damage is reduced, and defeating a certain number of enemies will yield various special bonuses (such as increase the number of Rupees dropped, to the appearance of rare Materials and/or Weapons).
It is possible to keep Focus Spirit going by collecting Magic Jars to refill the Magic Gauge. Young Link can play his Ocarina to convert SP into MP (Young Link's Strong Attack) and can even use it while in Focus Spirit allowing him to effectively remain in Focus Spirit as long as he has SP to convert into MP.
Character Specific Effects
Certain characters such as Young Link and Ghirahim may briefly transform into alternate forms while in Focus Spirit, such as Young Link's Fierce Deity form or Ghirahim's final form from Skyward Sword. Others such as Link in his Hylian Sword & Master Sword movesets may gain special effects such as the ability to unleash slash waves from the his sword, increasing the range of his attacks.
Focus Spirit Attacks
Pressing X while under the effects of Focus Spirit completely consumes the Magic Gauge, allowing characters to use their unique Focus Spirit Attack that knocks opponents down and exposes the weak points of Giant Bosses and strong enemies like Moblins and Darknuts.
Focus Spirit ends when the Magic Gauge is empty, which causes the player to automatically perform a weaker Focus Spirit Attack, which signals the end of Focus Spirit.
Each characters weapon moveset has its own unique Focus Spirit Attack. For example, Princess Zelda will perform a triple magic spell attack using Nayru's Love, Farore's Wind, and Din's Fire as her Focus Spirit Attack for her Rapier moveset or Ganon will summon his Trident and attack his enemies with it.