It’s basically a big torque converter that uses water to create load. Some water brake absorbers (Super Flow) add water to increase the load and some other systems (I’m not sure which) take away water to get the load.
Ok, I typed that out and I’m not sure I said it correctly. I’ll say it this way.
The Super Flow has a servo load control valve that (in most cases) is controlled by a computer. And that valve can either reduce water expelled by the absorber to increase the load. That’s how Super Flow does it. If the absorber needs more load to control the acceleration rate (which is also controlled by the computer and the servo valve) then the controller on the servo closes and forces more water to stay in the absorber and that creates more load.
There are some systems (I can’t name them but I know they are out there) that dump water to increase the load.
You can control how fast or how slow the engine accelerates by that servo valve and the load controller that controls how much water is either retained or dumped by it.