Ann: Research & Development Claim Lodged, page-200

If they're using in-app purchase then on iPhone at least there's no need to take payment details - it would all come through the card associated with their iTunes account.

I would imagine the user would need to agree to the in-app purchase (subscription) when they first use the app but it would be delayed until the end of the trial period. If they don't cancel it through the phone's Settings -> Subscriptions before the end of the trial then they'll start paying every month/year/whatever. That's what's happened when I've subscribed to other services via in-app purchase.

No idea how it'll work on Android, I never use the things.