Quote from ARogueTrader:
My understanding is that the amount of money available in these pre-paid cards is not actually stored in the information on the card itself.
What happens is you buy a blank card that has a serial number on it. The magnetic card actually contains the serial number of the card, and maybe some anti-theft info.
They you "charge" the card, and the amount "charged" to the card (pre-paid) is stored on an internet gateway of a company like Shift4. The "charged" amount is the pre-paid amount of the card.
When you go to swipe or use the card, the internet transaction runs through the following steps.
1. The swipe is made or the number is inputed.
2. This info is sent via the internet to a server that tries to capture an amount requested from the account where the funds are stored (the pre-paid account).
3. The amount is deducted (if sufficient funds are available) and the balance post sale/capture is recorded for the next transaction. The sale is rejected if the amount of the sale exceeds the amount in the pre-paid account.
Not unlike a credit card, is it?