I just booked several flights and they showed €0.02 return, but, when I got the confirmation I had the following:
Payment Amount 0.02 EUR
Amount Paid 0.02 EUR
Amount Due 0.00 EUR
So they are truly free flights, of course you'll need to do on-line check-in, pay with Visa Electron, no baggage and no insurance.
Booked them like this before and there is no deduction from your credit card!
The "limited time promotion" for the Electron card has now been over 4 years, if my memory serves me right.