Sure I am by no means an expert on this but from what I understand it does the following:
With the lower volume sounds (bass and treble) it will apply an amplification to raise them up. This is not just a volume increase, it does do some changes to each range. Alongside this any higher volume sounds are reduced (again with a particular eq setting for the ranges). The result is that you get a normalised song that helps to balance out those wide range songs and sounds. You hear this as though bass is a little louder and treble or high pitched noises are a little quieter.
As far as I am aware this is the only EQ setting that does this based on dynamically altering the sounds rather than applying a standard change to each range of sounds.