What is the meaning of "Lecha Zero'a Im Gevurah"?
Radak: Everything stands with Your strength. This is like "umi'Tachas Zero'os Olam" (Devarim 33:27).
Malbim: Regarding Hashem, Zero'a refers to the primary strength that moves the hand that acts (just like in people, the upper arm moves the hand). A person's Zero'a is not his; the power that moves it is external to it, whether one considers the natural cause or Divine cause. All worldly motion has a cause external to the object. Your Zero'a is Yours. You are the First cause of everything. Similarly, a person's Gevurah is external to him. A person need not be a Gibor; if he is a Gibor, this is in addition to being a person. Your Gevurah is intrinsic to You. It is not an adjective, rather, You are Gevurah itself. [Human] Gevurah is relative; it is seen with respect to someone weaker than the Gibor. Your Gevurah is not relative to something else; there is nothing other than You, for You created everything! Your Gevurah is tied to Your Zero'a.
Why does it say "Ta'oz Yadecha Tarum Yeminecha"?
Malbim: Hashem's Gevurah is revealed via miracles that overpower nature. A parable for this is one who turns a mill with great strength using his left hand, and with his right hand he stops the left hand's motion and turns the wheel in the opposite direction. So Hashem turns the sphere of the Ma'arachah and nature with His left hand, and His right hand overrides this via miraculous conduct. His Gevurah is seen with respect to Himself (His left hand), and not with respect to something external to Him.
