Credit to Head For Points for posting this originally.
You input this *AFTER* you have selected your flights.
The discount works for the total fare including taxes, which is impressive.
It's valid on flights booked by Tuesday 28th February, for flights in March.
You must book a return flight on Iberia or Air Nostrum from the UK to Spain (all destinations), Panama, Guayaquil, Lima, Caracas, Cali, Medellin, Bogota, Johannesburg, Rio de Janeiro, Santiago de Chile or El Salvador.