FRatSTN
To retain the slots without using them they need to be handed back, at least 3 weeks before the date of operation.
If you don’t hand them back by that deadline, you’re classed as not having operated them, and will be subject to the normal rules of having them removed. The hand-back requirement is so the slots can be re-allocated as a series or as ad-hoc slots to another operator while the historic operator isn’t using them.
The waiver also doesn’t apply to any operator who’s pulled out of an airport completely, so no waiver for Virgin Atlantic at LGW. Can’t retain historic slots at an airport with no flights operating at all.
The hand back rules allow launches such as the JetBlue at LHR that had previously been denied slots.
I think the above rules are important, they protect airlines dealing with reduced demand. But they also protect airports by opening up unused slots to other airlines. Otherwise incumbents would just sit on their slots, not use them, but the airport would be closed to new entrants due to there being no slots available!!