During a showdown, if I return my only defending unit to base, does my hidden card at that battlefield go to trash? Also, can I play 'Ride the Wind' to move a unit from my base to that battlefield as a defender?

Ruling: The hidden card does not go to trash because you still control the battlefield while the showdown is contested. You can play 'Ride the Wind' to move a unit from your base to the contested battlefield as a defender.

Sequence:

• Showdown begins when opponent attacks the battlefield
• Control cannot change while battlefield is contested
• Battlefield remains contested until showdown resolves
• Both players must pass priority with empty chain before damage
• After damage, showdown resolves
• Only then does control potentially change

Nuances:

• Even if all your units leave the battlefield during showdown, you maintain control until the showdown fully resolves
• Units can be added as defenders during the showdown using appropriate cards