So I took some pictures while watching the inauguration on my phone about a month ago.
During President Obama's last few hours in office, he showed a sign of dominance by intentionally guiding, soon to be President, Donald Trump.
Patting on the back and touching the arm may be an act of sympathy and friendship. It can also be an act of dominance. Again, between men and particularly in situations of power, this is a signal of who is in charge.
http://changingminds.org/techniques/body/touching.htm