Originally Posted by markmcfarland
Watch the video on their website. The shape of the exhaust creates a vacuum as water passes by. If the boat is not in motion ie revving engine in neutral them there may be some back pressure then. If you look at their graph on back pressure not until almost wot does the back pressure with an fae exceed one without. Back pressure only gets to around the 2 psi range. Running an engine on a hose will not simulate the vacuum that is created from the water passing by. There is another thread on here that says what this is called. It is something like "the vermillion effect". I know that's not the right word but its something like that.
I believe the word you are looking for is Venturi Effect