MEMtrim will trim the working set of a running process as a result, it will FREE up RAM memory for all other running applications. The working set of a process is the set of memory pages currently visible to the process in physical RAM.
Access to this memory is very fast because the pages are resident and available for an application to use without triggering a page fault. MEMtrim can explicitly trim the process working set.
This is essentially how the system trims the process when its top-level window is minimized. This does not mean that the memory pages used by the process are immediately discarded from RAM.
In fact, these pages may remain resident for quite a while. They are simply flagged so that the system can use them for other processes as necessary.