For a short explanation have a look at wholeness.
"The term PSYCHO-PHYSICAL is used both here and throughout my works to indicate the impossibility of separating 'physical' and 'mental' operations in our conceptions of the working of the human organism. [...] In my opinion the two must be considered entirely interdependent, and even more closely knit than is implied by such a phrase."  If you think Alexander's definition sounds complex, it certainly fits into his time.
Alfred Korzybski, a contemporary of Alexander, mentions 'psycho-physical unity' not verbatim, rather hidden yet utterly convinced: "The structure of the daily, as well as of the 'philosophical' language, which we inherited, in the main, from our primitive ancestors, is such that we have separate terms for factors which are not separable, such as 'matter', 'space', 'time', or 'body', 'soul', 'mind',. Then, as it were, we try to make out of the work, flesh, by reversing the natural order and affectively ascribing a delusional objectivity to these terms. If we deal with the silent, un-speakable, objective level and try to divide according to the implications of the verbal division, we find a brutal fact, which, until Einstein and Minkowski, has escaped scientific verbal formulation, that this cannot be done at all."
Both quotes were printed before the end of the Second World War. Korzybski and Alexander took great care of choosing their words to convey their ideas about the evolution of mankind. I don't know whether they knew of each other. Their concepts go together well, and illuminate aspects of human consciousness from seemingly different angles.
Translated into modern terms, Korzybski claims that the conclusions drawn from the relativity theory unveil the separation of body and mind as self-fulfilling prophecy. The structure of our language creates concepts of reality that distort our perception, the inability to conceptualize experiences in an integrative way drives us slowly insane.
Like Alexander, Korzybski stresses the importance of individual embodied experience. Alexander stumbled upon wholeness as a consequence of his experiments, Korzybski deducted wholeness from 'theoretical' grounds, both firmly assumed the usefulness of this idea for the basis of health.