Any effect which does not specify when it can be used may be used any time.
The exception is during Melee Exchanges or Ranged Attacks, where the timing is restricted as detailed in the Melee Exchanges and Ranged Attacks section.
Ki feats that state “when” they may be used can be used only when conditions are met. This can allow them to be used during melee exchanges and ranged attacks outside of the window normally allowed by those sections.
If both players wish to use such Effects the Active Player goes first.
If the trigger for using an effect is occurring while information is hidden then the effect may not be used.
Example
The Corruption of Virtue Event is used when an opponent uses a Virtue Token. If a Virtue Token is used, during a Melee Exchange to add more Special Ability options, this is designated Hidden Information. By the time the Player discovers the Virtue Token has been spent, the timing window for the trigger has passed and the Event cannot be used.
If an effect does not have a stated expiration time, it ends in the End Phase.