The idea is that instead of apache's fastcgi process manager exec'ing all the fastcgi processes, it can launch one, which can then load all of its shared data and then it can fork itself a bunch for the children.... most the memory pages will remain shared (containing perl, the code, the shared data), and as a result, more can be running without swapping.
The documentation is pretty sparse, but it's a damn good idea. See if you can write a simple script using FCGI::ProcManager and show me the apache configuration necessary to run it. It'd be very helpful.