Tom, I think the reason why you find Stringreen's metaphor a bit confusing is because, with all due respect to String, it IS a bit confused. I think possibly he is referring to an analogy between a high compliance cartridge and a lightweight, agile sports car, on one hand, and a low compliance cartridge and a truck, on the other. Think of the cantilevers on these two groups of cartridges as the spring suspension in the respective vehicles. The compliant sports car suspension will sag down to its bump stops with a heavy load, like that of a high effective mass tonearm, and vice-versa for a truck with a very light load, analogous to a low effective mass tonearm; it will start to fly around on turns and bumpy roads.
What I don't understand myself is why my high compliance Acutex cartridge (cu = 42 according to Acutex specs) and my high mass Fidelity Research FR64S tonearm, albeit with a light weight headshell, sound so good together, with none of the typical symptoms of a compliance/mass mismatch. I keep waiting for something bad about this match to make itself evident, but not so far.
