Can players play action spells back and forth after a chain is created but before the showdown resolves?

Ruling: Yes, players can play action spells back and forth before the showdown resolves, as long as they have mana.

Sequence:

• A chain is created (e.g., unit + action spell attack)
• Opponent can respond with their own action spell
• Players can continue playing action spells back and forth
• The showdown only resolves once both players pass in a row

Nuances:

• Passing indicates readiness to resolve ("I'm ready for this to resolve")
• If someone casts an action/reaction, everyone "un-readies" to give others a chance to respond
• The showdown/spell/ability only resolves once everyone is ready