What actions can an opponent take during your turn in an open gamestate?

Ruling: Opponents can only react when a chain is started. Playing units or gear alone does not start a chain and does not allow opponent reactions.

Sequence:

• Playing a unit/gear without a trigger: opponent cannot react
• Playing a spell: opponent can react
• Permanent trigger ability goes on chain: opponent can react
• Unit moves to unoccupied or enemy-controlled battlefield (showdown): opponent can react

Nuances:

• If a unit has "when you play me" text, this creates a trigger ability that starts a chain, allowing opponent reactions
• Unlike some other card games, not every action in open gamestate gives the opponent an opportunity to respond