Kundera's technique is unusual in that he's willing to mix so many different kinds of writing in one work. As we discuss in "Genre," Unbearable Lightness is a cross between fiction and philosophy, not to mention its historical and political content. In an experimental manner, the text jumps between different characters, plotlines, perspectives, and temporal settings.
Despite the non-linear narration, there is a strong sense of continuity throughout the novel. Part of this has to do with the thematic connections between the different plotlines (see discussions of lightness and weight in "What's Up with the Title?" or of es muss sein in "Symbolism, Imagery, Allegory"), and part of it has to do with the specific words and phrases that Kundera iterates again and again. To put it simply: he likes to repeat himself.
For example, Tomas first imagines Tereza as "a child someone had put in a bulrush basket daubed with pitch and sent down-stream for Tomas to fetch at the riverbank of his bed" (1.3.3). A few paragraphs later, it again occurs to Tomas that she is "child whom he had taken from a bulrush basket that had been daubed with pitch and sent to the riverbank of his bed" (1.3.9). In the next chapter, Tereza is "a child put in a pitch-daubed bulrush basket and sent downstream (1.4.11). You get the idea. When these specific words – "pitch-daubed," "bulrush," "sent downstream" come up again 200 pages later during Tomas's conversation with his son, we as readers immediately remember back to this first instance at the start of the novel. Kundera uses the same strategy with many other phrases to achieve an overall stylistic and strategic effect.