The source of “the shadow” can be stated clearly. AT: “Do you want Yahweh to make the shadow caused by the sunlight on the stairway to go forward ten steps or to go back ten steps?” (See: rc://en/ta/man/translate/figs-explicit)
This expression refers to the “stairway of Ahaz” in 2 Kings 20:11. This was probably a special stairway built for King Ahaz in such a way that its steps marked the hours of daylight as the sun’s rays advanced along them. In this way, the staircase served to tell the time during the day.