Charles H. Spurgeon on Psalms 110:6

The heads. Rather, the head; doubtless, the head of the Old Serpent (according to the prophecy in Ge 3:15), who acts in all who resist Christ. The verb "machats", which is used here, is employed to describe the prophetical and typical act of Jael, smiting the head of God's enemy, Sisera (Jud 5:26 4:22); and it is used in Ps 68:21, which describes Christ's victory, "God shall wound the head of his enemies"; and also by Hab 3:13, "Thou woundest the head out of the house of the wicked." —Christopher Wordsworth.