How does a free-up RAM utility free up RAM?

Basically what RAM free'ers do is that they reserve large amounts of RAM forcing Windows to swap other programs to disk and then freeing the RAM they reserved. There might be different strategies to do this, but I'm not awere of the internals.

I'd suggest to get more physical RAM if that's possible, as there will come the time when the outswapped program needs to get its share of memory back...

By trying to fool the operating system, it gives the impression that maybe something is being done. Such an approach is pretty much a case of robbing Peter to pay Paul.

Or... if you try and get something for nothing in the long run you get... nothing.

In other words, I regard this kind of thing as a con. If you are short of RAM, the best long term technical solution is to buy more.

Along with the other answers, older tools like RAMDoubler used to compress RAM in memory, with the obvious CPU cost on access. I think no one does this anymore, because of that access cost.

EDIT: no one.. well outside of Apple Everything old is new again. More specifically, the spread between CPU speed and speed of memory technologies (especially disk) is sooooo big that this is now a better option, especially with the electricity (read: battery) cost for hitting disk.

Windows sends a 'LOW_MEMORY' message around that asks all application to free their unused data. SQL server and file caches get cleared so you'll end up with lots of extra free space. This also clears the windows file cache wich can take easily take 1 gig of space on startup.

Allocating a big chunk of memory is an easy way to trigger a LOW MEMORY message but there might be smarter systems in place.