When ST4 analyzes a song, it runs through the entire track to figure out how loud it sounds on average, much like listening for the overall volume rather than the loudest spike; it then compares that loudness against the app’s preferred level of –14 dB and tells the player how much to nudge the volume so the song sits comfortably with others, but only boosts it by up to 6 dB to avoid making a quiet track suddenly blare.
A very simple variant of ReplayGain that leads to surprisingly similar results compared with ReplayGain.