From a dramatic perspective, the purpose of the scene is cast strong doubt on the idea that "nothing of the host survives." Up to that point in the show, this question (and specifically whether Sha're had survived) was one of the driving questions of the show. And in the very next episode, "Thor's Hammer," Daniel gets conclusive proof that the host does survive and can even influence the goa'uld's decisions.
Certainly, Amaunet is ultimately the one in charge as this event happens. However, we know that Sha're is able to influence Amaunet's actions to a certain extent. She manages to encode a message to her husband while Amaunet is torturing him in "Forever in a Day." A reasonable inference is that Sha're is trying to help out.
Of course, Sha're's help would be of no use if Amaunet could not be convinced to cooperate. Her specific personal motivations to do so are unknown, but the goa'uld leaders are all selfish megalomaniacs. She may not have much personal loyalty to Apophis, and keeping the SGC strong may give her a tool to use against him in the future. In "The Serpent's Lair," Bra'tac points out that a system lord often had the most to fear from their immediate kin, and Sha're could take momentary advantage of Amaunet's own natural plotting against her husband to cause the goa'uld to make an ill-advised split-second decision.