No worries guys I asked myself that same question. A SCWD is known also as a SQUID. Its a type of plumbing that kinda works like a wave maker. Its pretty much hooked up to your pump. The water is spit from the pump into hosing. It travels half the distance to ur tank and enter the SCWD. The SCWD has 2 other hoses connected to it leading to two seperate flanges (drainage into your tank). The benifit of this is that both of ur flanges are not fed water at the same time. Pressure builds up in the chamber causing it to switch from one flange to the other. In my set up it switches every 22 seconds. So one sec its putting water on one end of the tank and 22 sec later it goes to the other end. It pretty much helps out circulation in ur tank.
Its a cheap way to have a wave maker.
No electricity is involved.
If your satisfied with its circulation like I am you don't need a power head in the main tank.
A little extra plumming at start (but its easy)
Due to gravity you lose some gph
so you may need to upgrade or get a stronger pump.
You'll open the box and see the thing and say "I paid 30-40 bucks for this thing?" but its well worth it! GL