How does Yasuo's 'When I Attack' ability resolve when he moves into a battlefield, and can you respond with action spells or only reaction spells?

Ruling: When a unit moves to a battlefield and triggers a showdown, all 'When I Attack/Defend' abilities go on an initial chain at the beginning of the showdown. Players can only respond to this initial chain with reactions, not actions.

Sequence:

• Unit moves to battlefield, triggering a showdown
• Initial chain is created with all 'When I Attack/Defend' abilities
• Players can respond to this chain with reactions only
• Once the initial chain fully resolves, the attacking player gets priority and can cast actions first
• After that, both players can play actions

Nuances:

• Actions can only be played when the state is open (no active chain)
• The attacker gets to play actions first in showdowns after the initial chain resolves