World Line Shift

Divergence on inactive world lines is fixed and everything follows its predetermined path. Now we come to the present moment of the active world line, where divergence can be changed.

In concrete terms, it is possible to contradict the predetermined events of the currently active world line. The universe resolves this potential paradox by choosing a different world line to become active, out of the infinite superposition of inactive world lines. Physical reality is reconstructed accordingly. "Titor: 'When the world line changes, every cause and effect is reconstructed from past to future in order to prevent paradoxes.' — Chapter 2, Steins;Gate"

How Does This Work?
Suppose that it is written in the history of the current world line that Okabe absolutely will drink Dr. Pepper on a certain day. (Perhaps a time traveller has already confirmed this for you, or you have a God’s-eye view of the world line structure.) For some reason, he instead chooses to drink Coke. This is a contradiction, so the current world line must change to one inwhich it was predetermined that Okabe would drink Coke. On a sufficiently sensitive meter, a divergence change is detected.

World Line Is not Shifting to Just any World Line
In particular, the world line after a change should be the one that is most similar (in events, or divergence) to the previously active world line, out of those that are consistent with the newly created present. In many cases, this means that the past (and future) can remain largely unchanged. However, that assumes the existence of appropriate world lines. If Okabe was actually born with a genetic predisposition to choose a favourite drink and only ever drink that one drink, then there is no such world line where he drinks Dr. Pepper one day and Coke the next. By changing his choice on one day, history would be forced to reconstruct such that Okabe had always drank Coke, and always will.