Funny you ask "what it is". My best effort of answering that question was, ( and of course the answer made sense for me only, not necessarily to someone else) this. It seems to me that it measures force (in physical sense) which you use for several types of activity (emotional, physical etc). People train with age and experience, and that can explain why boundaries between types are so blurred. But if, as I suspect, it all comes down to how many synaptic connections we've got to emotional core and logical core for example, then it just shows the speed at which we digest different types of information.