What is "ba'Ma'alos Asher Yardah"?
Radak: [The shade had descended 10 steps; it was now 10 hours of the day. It went back] the [10] steps that it had descended, [to the first step, indicating the first hour of the day].
What is "Asher Yardah v'Ma'alos Achaz"?
Rashi (from Sanhedrin 96a): The day that Achaz died, the sun descended quickly, 10 hours earlier than usual, so they would not eulogize him. Now they went back, and they were added to the day that Chizkiyah healed.
Radak: Ma'alos Achaz are steps (Malbim Yeshayah 38:8 - a Kli) that Achaz made, on which the shade indicates the hour of the day, like a sundial.
Did Chizkiyah need such a great sign? In any case, he will see on the third day that he goes to Beis Hashem!
Malbim: The sign was primarily for the nations. Sancheriv had cast them off to the ends of the land. Hashem did not want them to perish among the wolves (other nations). Due to this sign, they feared Yisrael. Some Jews who had been exiled were strengthened, and returned to live in the shade of Chizkiyah. Had they merited, this would have been the ultimate gathering of exiles (Chizkiyah would have been Mashi'ach).
