The venturi in this case would be where the bend is, for the sake of using the same picture above. In the lower picture to the left hand side is the suction side. The trash pump's discharge is not shown but connects at the bend where the smaller pipe in the picture double backs on itself. Wait one let me find a better picture for the ahhh haa moment.
Ok now the water coming in will be pressurized that water once it dissipates slightly to it's new opening size will also have to draw extra water in to try and "equalize" itself.
Airlift's do also use a venturi effect only in a different way.
This gentleman is better at explaining it than I am.
https://www.youtube.com/watch?v=Na9ORhYjvJUApparently embeded videos are broken so just posting the links now.
