You've just got
to go asking good questions, don't you?
So here's what happens with a spring lever:
-You start the extraction by pulling down the lever, compressing the spring
-As the shot progresses, the spring un-compresses and exerts less and less downward pressure on the piston
-This ^^^ coincides with solids eroding out of the puck as the shot progresses, which would normally increase flow... IF pressure on the piston remains constant. BUT on a spring lever, the pressure on the piston doesn't remain the same. It decreases as the shot progresses. This has the "effect" of keeping the flow constant, because as the puck erodes, it takes less and less pressure
to keep the same
flow. I used quotes on "effect" because the declining pressure on a spring lever may or may not exactly
match the rate of puck erosion.
On a spring lever, you have no control of the rate of declining pressure, except by swapping springs. In racing cars and downhill mountain bikes, spring swapping is part of tuning. I don't know if the makers of spring levers make a variety of springs to "tune" with.
But this is a thread
about using water debit to adjust brew parameters, which implies the ability to set any water debit you want, on the fly. (Something that you could easily rig your BDB to do, BTW).