"His (Shimon's) hands did battle for him," as it is written (Bereshith 34:25) "And there took, two sons of Jacob, Shimon and Levi, etc." "And You shall be a help against his foes": as in (Ibid. 35:5) "And they journeyed, and the terror of G-d was on the cities around them, and they did not pursue the sons of Jacob."
Variantly: "And this for Judah": Moses prayed for David, saying before Him, "L-rd of the world, whenever the kings of the house of David (descendents of Judah) are in distress and pray before You, rescue them from it."