I know how to operate the driver variable speed limiter, but how it works is a different question! The device is part of the cruise control system which itself is linked to the automatic transmission or its software at least. These devices are available on a great many new cars and are probably bought in from a supplier such as Bosch or Continental, like the ASR and ESP systems. Try the former's website, often there is quite extensive technical information available.
You may also wish to consider the distronic system fitted to the new S-Class and CL cars. This is a cruise control system which maintains a constant distance from traffic ahead by cutting engine power and even applying up to 1/3 of maximum braking power. This too is a bought in system as it is available on Jaguar's XKR.
If you have a Mercedes, maybe your dealer/service technician would be happy to talk to you. I wonder what the MB maintenance CD-roms available on this website would tell you.