Similar to many Mastercard/Visa cards. The electronic chip on the card is capable of actually storing the balance and then syncing it back when it reaches a terminal machine that’s connected to the internet.

In my experience, many (most) chip-card based transport systems for busses or trains (eg tap n go) operates in this offline mode where it only syncs when you go to the kiosk to top it up.

It uses cryptographic methods to securely store and update the balance. The chip itself is an embedded processor.