From the Skywards Website:
If you are a Gold member with a Skywards Flex Fare commercial ticket, in any class, with no minimum/maximum stay restrictions*, we will guarantee an Economy Class seat for you if no tickets are available in any other class on your flight. This service is only available if you have given us your Emirates e-ticket number and made your reservation at least 48 hours before the flight. You will also need to send a copy of your ticket to the nearest Emirates Contact Centre. On the day of your flight, please make sure you check in at least 90 minutes before the flight departs to guarantee your seat.
If you are a Platinum member with a a Skywards Flex Fare commercial ticket, in any class, with no minimum/maximum stay restrictions*, we will guarantee a Business or Economy Class seat for you if no tickets are available in any other class on your flight. This service is only available if you have given us your Emirates e-ticket number and made your reservation at least 48 hours before the flight. You will also need to send a copy of your ticket to the nearest Emirates Contact Centre. On the day of your flight, please make sure you check in at least 90 minutes before the flight departs to guarantee your seat.
I'm surprised at this as well. Didn't look into it until now and assumed it was a guaranteed ticket in Y if Y was sold out, not the entire flight.