Malbim: It is about straightness. It uses the metaphor of baring the Kadosh arm and speaking exalted matters, as if straightness is a living entity and the author of this song. It tells the great power of straightness. It seems that Asaf composed it in the days of Yehoshafat. Then there was affliction from the enemy that came against them. In Yehoshafat's merit, that he established judges in every city to do Mishpat and straightness, they were saved. They saw the great power of straightness!
2)
Why does it say "Al Tashches"?
1.
Rashi: Yisrael should not be destroyed.
2.
Malbim: This hints that the enemy was afflicting them (refer to 75:1:1:2).
