Complex sentence: To avoid the problem of fare-cheats, ie people who pay the wrong fare, the system is based on a maximum deduction at the start and then a refund when the card is tapped again at the exit.
Split into simpler sentences: To avoid the problem of fare-cheats, i.e. people who pay the wrong fare. The system imposes a maximum deduction at the start and then a refund when the card is tapped again at the exit.