Welcome back the the salt world! and TRT
yes the CLS is a water movement thing. PVC pipe and a pump.
you can do it many ways, drilling the tank and using bulkheads, or doing the up and over /down to the pump way.
by using the CLS you take out the need for the power heads sitting in your tank( and adding heat to it)
since it is a seperate system then the main sump, you can keep it going during any work in the sump. also there shoudl not be any priming for it if you lose power to the system.
This is one that is an over the top and requires no drilling. Alot of people drill their tank to get a CLS. Look af Geoff's build page. I think he drilled his tank. He has done a lot of cool stuff to his tank.
Heres a pic of one i just built. Its a little extreme but every pipe you see, with the exception of the straight one across the top, is for the closed loop system. The bigger 2 lines meeting and going off to the left is the pump infeed(pump is in the stand you cant see it, and the other 8 lines feed 2 spraybars in the bottom of the tank and the 6 lines all going different directions in the top. The way this setup works is that 3 of those 8 lines are always moving water, and which 3 changes every 30 seconds. Gives it a more natural reef current.