Can hidden reaction spells be played at any time like normal reactions, or only when reacting to specific game events?

Ruling: Hidden cards can be played any time you could play a reaction, starting with the next turn after hiding them.

Nuances:

• Reactions can only be played during windows created by showdowns, spells being played, or abilities being used
• Playing permanents (units and gears) in an open state does not create a reaction window
• "When you play me" triggers go on the Chain and create a reaction window
• Playing another unit that triggers "when you play another unit" abilities also creates a reaction window