Does Tideturner work with cards in hand or only cards on the battlefield? Also, if Yasuo is swapped into a showdown using Tideturner, does his 'on attack' trigger?

Ruling: Tideturner only works with cards already on the battlefield, not cards in hand. When a unit with an Attack or Defend trigger enters combat mid-showdown via Tideturner, that trigger goes on the chain only if appropriate for their attacker/defender status.

Sequence:

• Attacker/defender designations are applied to players first
• Units get assigned during the showdown process
• When Tideturner resolves and swaps a unit in, that unit gets assigned defender during Tideturner's cleanup
• If the swapped-in unit has an Attack or Defend trigger, it only triggers if their role (attacker/defender) matches the trigger condition

Nuances:

• In the Yasuo example, since Yasuo's controller is not the attacker, Yasuo's "on attack" trigger would not activate
• Tideturner received errata so that when hidden, it can now target another battlefield (previously it couldn't)