ESP metering mode is the Olympus equivalent of matrix metering modes found on other cameras
essentially it compares the pattern of 324 segments of the image with an image database to determine the most likely correct exposure, HOWEVER, if Face Detection AF is on (even in MF mode), if it detects a face, the metering will CHANGE to meter the face ONLY!
for the most part then, the algorithm used is rather like black magic to the user, but nevertheless for most situations works very well.
Unfortunately, it still tries to make things into 18% gray scale and does not really understand what you are shooting nor how YOU want it rendered.
THUS, for many situations you will need to resort to exposure compensation or spot meter modes of Olympus cameras