What does iPhone "Sound Check" feature do?

It's supposed to "level out" the volume of the audio, so if you've ripped one track from a source with a low volume level it shouldn't be as noticeable if it is then followed by a really loud track. However, it's very hit and miss. From experience, it likes making my Paul McCartney tracks even quieter (and for some reason the transfers used for his remastered albums were quiet compared to other albums to begin with) then normal.