Why does it say "Tizrem"?
Rashi: You will winnow them like with a pitchfork.
Radak, Malbim: You will winnow them like one who winnows grain after threshing, and the wind takes the chaff. They will be like the chaff.
To where will wind take them?
Rashi: To Gehinom.
Malbim: A small wind will take them high in the air. A Se'arah (gust) will scatter them in every direction.
Why will you rejoice in Hashem?
Malbim: Due His great strength.
What is the meaning of "bi'Kdosh Yisrael Tis'halel"?
Malbim: You will boast to the Rabim that His Kedushah is attributed to you, via your deeds. One can rejoice in success due to an external cause, but he can boast only if it is due to his deed and skill.
