What happens if a Power that references an Incarna is used by a Spirit that doesn't have that Incarna (e.g., if it's transferred via Entwined Power)?

What happens if a Power that references an Incarna is used by a Spirit that doesn't have that Incarna (e.g., if it's transferred via Entwined Power)?

Resolve the Power exactly as written. Though an Incarna is associated with a particular Spirit, it's still a piece that other Spirits can interact with — they just don't normally have a way to do so.
Depending on the Power and Incarna in question, this may be weaker than when used by the original Spirit. For example, while Frightful Keening can still push the Incarna of Wandering Voice Keens Delirium, and still generates Fear if pushing into a land with Invaders, only Wandering Voice Keens Delirium can use this Power to add Strife via Keening Spurs Grievances.