Can you activate a Hidden card from battlefield B while a showdown is happening on battlefield A?

Ruling: Yes, you can activate a Hidden card from one battlefield while a showdown is happening on another battlefield. It works the same as playing a reaction card from hand, as long as it's legal to play in that instance.

Nuances:

• You cannot cross-target between battlefields. For example, you cannot use a Hidden Blade that's face-down at battlefield B to target a unit at battlefield A.
• Hidden spells and play effects of hidden permanents must target options at the same battlefield where they were played from, unless the ability explicitly restricts targeting in a way that makes this impossible (like Tideturner).
• Hidden units must be played to the battlefield they were revealed from.
• Non-targeting effects (like drawing cards) can still be used from hidden cards at one battlefield while action is happening at another.