What do we learn from "Regel Yesharah"?
Brachos 10b: One must put his feet together when praying. (It is as if angels have only one foot. Rashba - we emulate the angels during prayer, for one must try to emulate their perfect deeds. Additionally, one should consider himself unable to move without Hashem's help.)
Chagigah 13b: Since Yechezkel was able to see their feet, we infer that the wings that cover their feet were lost after the Churban 1 .
Rashi #1: They are in line with each other 2 .
Rashi #2: They do not have knees to bend their thighs, for they do not sit or lie. They do not need knees, like animals have upper and lower knees, in order to bend their thighs to lie down.
Malbim: The wings hint to when they fly above. Then, the Ofanim rise with them (19). I.e. they rise above the Ma'arachah, according to Hashem's desire. The legs hint to when they stand or go on the ground (conduct according to nature and the Ma'arachah). The ground under the Chayos is in the place of the Ofanim. For spiritual beings, Regel refers to what they cause. Their Regel is straight, i.e. when they conduct according to the Ma'arachah, the results are straight, i.e. according to the causes like was fixed from the beginning of creation. They have no knees, i.e. interruptions between cause and result.
13b: Others say that the wings used to sing were lost. Their feet became exposed, and Yechezkel saw them.
Perhaps the legs of one are in line with another's legs. It does not mean that their feet are straight and parallel, unlike human feet, for after this the verse discusses Kaf Regel (their feet)! (PF)
What is "Regel Egel"?
Rashi citing Targum Yonasan: It is a round foot.
Malbim: Feet stand on the ground. Their feet are on Olam ha'Ofanim, and they move them. It is a circular motion. They influence a fixed rotation; the Ofanim make the Galgalim rotate.
What are "Notzetzim"?
Rashi: Amidst their radiance, it seems that sparks come out from them. Notzetzim is like 'she made a gold Nivreshes? when the sun shines, Netzotzos exude from it' (Yuma 37b).
What is "k'Ein Nechoshes Kalal"?
Rashi: It appears like purified copper, and illuminates.
Rashi citing Menachem: This is like "Kaluy ba'Esh" (Vayikra 2:14; roasted).
Radak: It is Memurat (polished) and clear.
Radak citing Targum Yonasan: It shines like golden copper.
Malbim: Sparks come out of their feet like those of Nechoshes Kalal. This is in Olam ha'Asiah, of the Galgalim. There the Nachash, Nachash ha'Nechoshes, trampled. It is the physical world of composites. Just sparks come to it from the feet of the Chayos - the cleanest, clearest polished copper. Their influence is clear, opposite the cloudiness of this world. Still, it has a little filth of the Nachash - Eitz ha'Da'as Tov va'Ra, from which they ate due to the Nachash. Also in Olam ha'Yetzirah is a little [of this filth]; the influence descends below to living beings. A Midrash says that Kalal refers to burning. Since their purpose was to burn the Mikdash, he saw the sparks from them as fire from the Nachash that cast venom and consuming fire in the Mikdash.
