כִּ֭י תְּשִׁיתֵ֣מוֹ שֶׁ֑כֶם בְּ֝מֵֽיתָרֶ֗יךָ תְּכוֹנֵ֥ן עַל־פְּנֵיהֶֽם׃
For thou shalt make them turn their back, Thou shalt make ready with thy bowstrings against the face of them.
Read midrashic interpretation of Psalms 21:13: and classical rabbinic exegesis.