It was one of the more effective training exercises that I saw during my years in the New York Police Department.

The instructor would call up four cops, two black and two white, to the front of the class. He’d have one of the black cops face the wall with his hands up and place the two white cops close behind him, on either side, pretending to point guns at him.

The instructor would then ask the class, “Whadda we got?”

Everyone knew the answer: an arrest or a stop.

Then the instructor would switch the positions, arranging two black cops behind one of the whites. This time, white hands were raised in surrender, and black hands mimicked guns.