In The Witch of Blackbird Pond, why was the sentence about the townspeople's reaction to Kit so effective?

When Kit arrives at the Meeting House for the first time, the townspeople react with surprise, as they have never seen anyone like her before, yet they do not say anything.

"As Kit moved behind her the astonishment of the assembled townspeople met her with the impact of a gathering wave."

This is effective because it shows the shock of the people and the gathering tension of the moment. The townspeople are stunned. They are watching, drawing in, and holding their opinions inside themselves, but (like a wave) that tension will eventually release with a spectacular force. This foreshadows the moment when the townspeople release their frustrations and long-held opinions of Kit at the trial. All of the things that the townspeople were thinking at the moment of this quote are vented when she is accused of being a witch.