Oh, simple. Essentially I'm using a foam filter block from an aquaclear filter. I drop that in the sump in an area of flow, and over a few weeks, it becomes colonized with nitrifying bacteria. Some people use round foam filters, bio-balls, excess LR rubble. any material with a large surface area that can hold a lot of bacteria.
Then, in my case, should I need to set up a Q tank or treatment tank, I can add the foam block to my aquaclear and presto, a fully mature (cycled) filter to remove NH4 and NO2 from the tank.
Some foam filters are air driven, some fit in canisters, etc
And it's in the sump just to keep it out of site. You could float it around in your tank
It's not specifically a sump thing...