Use once as a free action: if your queen still stands on its home square (d1 for white) and your bishop on c1, they swap places.
You draft Royal Barter in either mode. Free action: you choose when to use it, and it resolves without costing your turn.
As a item card, it is a one-use consumable, drafted in either mode.
“Her majesty trades pews with the c-file clergy.”