Kevin:
A bi-directional pump is the equivalent to a ram pump.
The unit does not behave like a ram pump, as a principle, it has two pistons, with an area ratio high enough to raise the water to the upper tank.
25 feet drop and 65 feet raising, the ratio is 65/25 = 2.6 and to correct for frictions this ratio is increased like 4 to 1, but due to how the water is fed to the pushing tank the volume becomes 5 to 1, one (1) for pushing and four (4) for pressure that goes down hill to the ground, if the system is using the same flow for pressure and pushing.
So the pressure piston, which is filed by the spring, is 4 times larger area than the raisin piston to bring the water to the tank.
The pressure pipe fills and when with enough head, the water in the second pushing piston feeds the pipe going to the upper tank, in this case the water volume delivered would be 1/4 of the water available.
Bi-directional pumping for easier mechanical set up, which is done by the piston's position to change the water direction feed in the pressure piston, the pushing piston has 4 one way valves to allow flow in/out automatically.
The critical part is the directional water valve feeding the two sides of the pressure pipe in sequence for proper directional pressure cycling.
Nando