Black Tank Flush

[NYMoose]

I have a 2005 340bhs, and I got from the factory there black tank flush. ($80) I hook a hose up to it, and I have to admit, I dont hear or see much going on. I figured I would hear somewhat of a high pressure water sound in the black tank?? Anyone else have one installed, and how does your work? My tailer is goign back to the dealer for a few things to be repaired while under warranty, and this is on the list to be looked at. It just does not seem right to me. While black tank valve is open, only small amount of water is coming out??