When can you use REACT spells in response to opponent's abilities, spells, or gear outside of showdown phase?

Ruling: You cannot react to gear or units as they are played. When spells, triggered abilities, or activated abilities are played, they create a chain which allows you to play reaction spells even outside of showdown. Reaction spells can be played in all game states (Neutral/Showdown and Open/Closed).

Sequence:

• Opponent plays a spell, triggered ability, or activated ability
• A chain is formed (Closed state)
• You can now respond with reaction spells
• This works even outside of showdown (Neutral state)

Nuances:

• Action spells cannot be played during chains; they can only start chains
• Action spells can only be played during Showdown or Neutral Open states
• Base spells can only be played during Neutral Open states
• Anything that says "add" does not create a chain and cannot be reacted to
• Units played without triggered abilities cannot be reacted to before they move to battlefield