There aren't as many examples of flashback in this novel as there are of foreshadowing, as your previous question focussed on. However, we have definite flashbacks in Chapter 15, when David is reunited with Sophie after so long apart. One of these occurs when Sophie expresses her absolute desolation at what has been done to her because she was a deviant. Seeing her crying triggers a flashback in David's memory:
I remembered watching. The man with his arm linked in the woman's, the small figure on top of the pack-horse waving back to me as they disappeared into the trees. Myself desolate, a kiss still damp on my cheek, a lock tied with a yellow ribbon in my hand. I looked at her now, and my heart ached.
Note that this flashback serves to heighten and emphasise the horror that Sophie has experienced by remembering her as a fragile, vulnerable child. Seeing the older Sophie grieving over her forced sterilisation juxtaposed with the child Sophie, David's friend, exacerbates the difference and demonstrates the tremendous cruelty of the Waknuk regime.