Another way of "looking" at it (he he) is if you stared at a UV light for about 1 second you would not only get the worst sunburn in history, but you would also fry your retina's and go blind. As, GDominy mentioned, the slower the flow through the sterilizer, the more "contact time" there is with any algae spores or parasites. Basically it will disrupt the cells and either 1) mutate them so they are useless and can no longer reproduce or 2) fry them dead. I think if you do a search you'll find that most of us are using the Coralife turbo twist with varying lengths and wattage. I have a 9 watt and have my filter outtake split to a ball valve to have a flow less than 50 gph through it for my 46 g tank. The bigger the tank, the more wattage you'll need.
40g AGA Breeder: Pimp# 44: Eheim 2026; Anti- Filstar; CLTT UV; 3 wpg PC, pressurized CO2. GATA! FTD!!!