The table below presents an aligned extract of verses or verselets in Gn15-17b Gn15-17c Both verses/verselets discuss nightfall. The alignment justifies the Rashi comment that: The sun sank refers to sunset while there was steathliness refers to Nightfall. More probably this symbolizes that the destruction of the nations of power - symbolized by furance fire that devoured the power animals - would not occur till the Jews were totally helpless symbolized by the setting of the sun and the fall of total nightfall. And just then when the Jews had lost all light and were confounded in stealth, just then God would send a furnace fire to devour the nations of power that had attacked us.

Verse	Text of Verse	Rashi comment
Gn15-17	And it came to pass, that, when the sun sank, [Sunset]and there was steathliness, [nightfall] behold a smoking furnace, and a flaming torch that passed between these pieces.	The sun sank refers to sunset while there was steathliness refers to Nightfall. More probably this symbolizes that the destruction of the nations of power - symbolized by furance fire that devoured the power animals - would not occur till the Jews were totally helpless symbolized by the setting of the sun and the fall of total nightfall. And just then when the Jews had lost all light and were confounded in stealth, just then God would send a furnace fire to devour the nations of power that had attacked us.
Gn15-17	And it came to pass, that, when the sun sank, [Sunset]and there was steathliness, [nightfall] behold a smoking furnace, and a flaming torch that passed between these pieces.