Where you take water out of the tank from one point, pass it through a pump and retrurn it to the tank in other spots like Loc Line nozzles for circulation. The water is never exposed to atmosphere like in a sump, its always enclosed so called a closed loop. I used one for 7 years where I passed water through a 1" bulkhead in the center back of my display, passed it through an Ocean Runner 3500 pump and returned it to the two upper back corners through a Oceans Motions Squirt two way and Loc Line nozzles so I got nice random flows and extra circulation without having powerheads visible in the tank. The drawback is a closed loop pump usually draws more power than something like two Korals Evolutions powerheads.