Digital Persistence is a feature in all RC4 dimmers.  When used with LED light sources, it emulates the way a traditional tungsten lamp behaves when power is applied and removed.  The filament in those lamps take time to heat up and produce white-hot light output.  Similarly, they take time to cool off and stop producing visible light.  The larger the bulb or lamp, the longer this takes.  The characteristics of these rises and falls are very familiar and are not linear.  Digital Persistence with RC4 dimmers provide more natural looking LED brightness transitions, with a range of 8 speeds to choose from.