Basic reference
Pagat’s base version requires a player who cannot follow suit to ask for trump to be exposed. After exposure, trump can beat other suits. A player unable to follow suit is not automatically obliged to trump in that base version.
A meaningful variation
The same reference describes versions in which asking is optional, but the player who asks must trump if possible. Those instructions are not interchangeable.
Before your first hand
Write your reveal rule beside the app mode or reference title. During practice, pause when someone is unable to follow suit and identify which rule applies. Check the linked source for all exceptions.