With most toilets, a reservoir sits above the main bowl. This reservoir fills at the time of the flush, and remains full (barring leaks) until again flushed. This reservoir is commonly referred to as the upper deck.
When it becomes necessary to make a deposit in the tank of the toilet, circumstances do not always allow for proper checking of water pressure. While a sink won't run without water pressure, a toilet will often allow one last flush. Use of this flush to remove an especially unpleasant deposit is reffered to as an upper deck save.