[Parker]

My thoughts behind the question:

Traditional overflows have air introduced into them to quiet them down. The overflow works on a weir type principle ( might be the wrong term ) Water flows over the edge of the pipe (HGB) and up through the pipe ( Duroso ) Since air is introduced into the system you have air mixed with water traveling down the pipe. With a herbie no air is introduced into the pipe, therefore space previously occupied by air is now being taken up by water. On top of that you are also introducing head pressure, how much of a difference that makes is up in the air.