Why does it say "Atah Yadati..."?
Rashi: This is what we will sing now for the salvation that came to Yo'av and Yisrael. I know that Hashem desired me and answered me from His Kadosh Shamayim, for their salvation is my salvation.
Radak: Everyone in Yisrael will sing so. Now that Hashem will save David, all will know that he is Hashem's Mashi'ach (He desires him), unlike his enemies had said that he is a sinner, and He does not desire him - "Ein Yeshu'asah Lo vEi'lokim" (3:3).
Malbim: Now I clearly know that Hashem answered His Mashi'ach from His Kodesh Shamayim.
Why does it say "bi'Gevuros Yesha Yemino"?
Radak: With a small Am, he felled myriads of his enemies. He could do so only via the might of Hashem's right hand that He sent to save him.
Malbim: I know that Hashem answered him, for the salvation was through Gevurah attributed to His right hand, above nature.
