What version of WHM do you use? I don't see this option to exist in the latest Release. However i suspect that this option just "unloads" some not frequently used programs from memory (whostmgr is a candidate) but in the penalty that every time you need to use the program you have to "load" it again which causes more CPU usage and Disk I/O.
This is a new feature and has not propagated to all the builds yet.
It optimizes the way memory is allocated (cutting on number of child processes etc.). However, by doing this, it is very likely that more CPU/DiskIO resources will be used. This is designed for those on VPS servers. If you are on a dedicated server, you probably shouldn't use this (for performance reasons).
