The "lock to reduce the cost" effect can be used when the action is announced or when the cost is paid.

They can lock to use their ability the turn they where recruited.

Can be locked to reduce the cost at any point before resolution, from when the action is announced to just before the cost is paid. If it is used when announcing an action, and the action is cancelled as played (e.g. Direct Intervention), it is not locked.

Can reduce the cost of an action card even if it is not played as an action (e.g. a weapon played with Concealed Weapon or an Ally with Piper).

Allows to play a card that could otherwise not be afforded, in which case, it must be locked when the action is announced.