This detail of the narrative may be read in connection with Deut. 33:20. In this verse and the following verses, Moses prophesied concerning the Tribe of Gad, among other things, "He dwelleth as a lion and teareth the arm with the crown of the head." That means, according to Hebrew commentators, that the warriors of that Tribe had learned to disarm, and kill an antagonist by smiting the crown of his head and his arm with one stroke, or one movement of the sword's blade.
Ammon seems to have learned this manner of swordsplay, in which the warriors of Gad excelled, and which inspired Moses to compare that Tribe to a lion having its lair in the wilderness.
For the arms of the wicked shall be broken: but the LORD upholdeth the righteous.