Why does the general refer to the pace of the boy's drumming as "the heart of the army" in "The Drummer Boy of Dhiloh?

He needs Joby to set a pace with his drum so that the men will fight with a strong spirit. "If he, Joby, beat slow tomorrow, the heart would beat slow in the men." "But if he beat a sure, steady, ever fast rhythm, then, then their knees will come up in a long line down over that hill ..." "He gave the orders, but Joby set the pace." The beat is a controlling force; it sets the tone of war.