Rashi: From the beginning, there was a sign between Yisrael and the ambush, that the ambush will make smoke come from the city, and Yisrael will know that they conquered Giv'ah [and they will return to fight Binyamin].

Rashi: It is the height of the smoke.

Radak: Mas'as refers to burning, like "va'Yisa'em David" (Shmuel 2:5:21). Mas'as refers to the flame, which rises above. A Mishnah (Rosh Hashanah 2:2) says 'Mesi'in Masa'os.' The word Mas'as refers also to the smoke, which rises. It is as it says Mas'as ha'Lahav u'Mas'as ha'Ashan; the word Mas'as is in place of two. There should be a Vov in front of ha'Ashan; it is omitted.