So, what is regressive suspension ? This suspension reduces it hardness if the load/weight increases. It softens the strikes of wheels more effective. Because if wheel strike heavier , then hardness of suspension reduses. Which is natural. In opposite to widely used progressive suspension, which increases it's hardness and make strikes more heavy.
So, what's my idea. Look at Fig. 1. AB, AC - rods. BC is spring and always horizontal. Vertical force in A point depends by cos(ABD). × cos(ACD) (1) . Which is nonlinear and represents this regressiveness: more force - less hardness. If C', D' not equal to C, D. Then (1) is more complex, but regressiveness saves anyway.
If AB is a spring, then regressiveness increases - Fig.2.
Very useful BC to be pneumatic or half pneumatic half still, for cars to control their height.