It is not clear how the stone would sink into his forehead. Perhaps it hit him on the side of his head to make a visible dent in the helmet he was wearing. This would then knock him unconscious for some time. The fact that Goliath ends up falling face down on the ground draws a parallel with the fall of the Philistine god Dagon (1 Samuel 5:3–4).1
49 Il mit la main dans sa gibecière, y prit une pierre, et la lança avec sa fronde; il frappa le Philistin au front, et la pierre s'enfonça dans le front du Philistin, qui tomba le visage contre terre.