The flashback style in All the King's Men is dazzling, like the highway mirages Jack warn of on the road to Mason City. It sometimes seems as if Jack can never move forward, because he is always brought back to the past. That the novel relies heavily on Jack's flashbacks and memories of the past reinforces this point. Even as he helps propel Willie into the future, Jack is lured by the past, by what he calls "that dark backwood and abysm of time" (emphasis ours; 1.153).
You might have noticed that this is a wry appropriation of Prospero's question to Miranda, in William Shakespeare's The Tempest, when she asks him about the past: "What see'st though else in the dark backward and abysm of time?" (1.2.13-14)
In other words, "What else do you remember?" Like many of the older people in All the King's Men, Prospero has past secrets he doesn't want to be made public.
In Jack's case, it is literally a backward journey into the backwoods. Backwoods are wooded areas that haven't been cut down or otherwise touched by human beings. It also means an area lived in by an community that has "backward" ideas – like heavy duty racism and corruption in the case of All the King's Men. We can see this in some form in nearly every flashback. In the case of the story of Cass Mastern in particular, what we witness is brutal and shocking.
By repeating elements of these flashbacks throughout the story, and by continually bringing up reconstructed images and sound bites, we get a freeze frame effect. That's what all the nicknames and defining phrases are all about (The Young Executive, Cousin Willie with the Christmas Tie, the Scholarly Attorney, etc.). Jack's mind freezes on these moments; they come to define the people behind them.
The image of Anne floating on her back with her eyes closed and the seagull flying overhead defines her in Jack's memory for a long time. This image is so powerful that he flashes on it after taking off all her clothes. This image prevents him from having sex with her, a decision that deals a blow to both their self-esteems and their relationship.
These freeze frames are also open to manipulation. Jack and others are fascinated with how changing someone's "picture of the world" can radically influence his or her behavior. The novel can be seen as a history of the changes in the pictures in Jack's head (and sometimes in the heads of other characters).