Why did he call Hashem "Magini"?
Radak: He saved me from Yishbi b'Nov.
Malbim: I did not need to fight at all.
How was he helped?
Radak: Avishai helped him - "va'Ya'azar Lo Avishai ben Tzeruyah" (Shmuel II, 21:17).
Why does it say that his heart rejoiced?
Radak: My intellect rejoiced, for my men swore that I will not go with them to war again; I was separated from worldly engagements, and I will have time to engage in Hashem's ways.
Why did he say "umi'Shiri Ahodenu"?
Rashi: The praise is "Hashem Oz Lamo" (verse 8) - for those who depend on Him, at the time when He is the strength of salvations for His anointed (David).
Radak: I will praise Him via my songs.
