What does Furmark Post-FX actually test?

[enigma]

Reason I am asking is that I have a prebuild desktop that I just got back from warranty repair after they replaced the graphics card after determining no issues with PSU (replaced), memory, OS, drivers, etc.
It works fine now.
Before I sent it in for repair I was having games crash (reboot computer with no errors in event logs or anything besides the usual computer was not shut down properly) if I tried to do anything after the loading screens (select a character, load a save, etc), and I tried a Furmark test at one point and the only time it did the same was when I turned on Post-FX.

Now I am paranoid about what the actual issue with the card was and if there was anything I could have done to prevent it or if it was just faulty. I had the computer about 6 weeks and all was fine up to it having this issue.

So what component of the graphics card does Post FX test, or what was it likely indicating was wrong with it exactly, seeing as how it does look to have been the card?