Reaction Command

Reaction Commands (リアクションコマンド) are a feature introduced in Kingdom Hearts II, intended to extend the level of combat. By pressing in specific contexts, the player character will perform a specific action indicated by the title of the command. These reactions are generally combat maneuvers used to gain the upper hand against enemies, though they are also used for more mundane actions such as talking to other characters and opening chests. Occasionally, some Reaction Commands must be completed in order to end a battle.

Only a select few Reaction Commands require the player to press the moment it appears; in Kingdom Hearts II, one can press  until the command is initiated, suffering no penalty if  is not pressed at the right moment. Only three Reaction Commands in the entirety of Kingdom Hearts II penalize such an action. In Kingdom Hearts Re:Chain of Memories, pressing too early during the execution of a Sleight prevents the attack from being completed.

In Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage- and Kingdom Hearts III, Reaction Commands are replaced by Situation Commands.

Kingdom Hearts
In the Kingdom Hearts HD 1.5 ReMIX version of Kingdom Hearts Final Mix, commands that appeared in the fourth command slot have been changed to appear as reaction commands separate from the menu, and overlap with the party member command button. As a result, the summon command has been shifted into the fourth slot.

Kingdom Hearts Chain of Memories
Note: Reaction commands only appear within the Kingdom Hearts Re:Chain of Memories'' version. Unlike in other games, the names of most of these commands do not show up when the command is available.''

Notes and references
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Commande Réaction Situations-Kommandos