In general, the snowball effect is defined as “A process that starts from an initial state of small significance and builds upon itself, becoming larger”. This effect is used in movies to create a sense of suspense and tension through cross-cutting.
For instance, it can increase the intensity of the final act with more than one conflict. If you use linear effect, then we get High (The first conflict) > Low (Transitioning to the second conflict) > Another High (The second conflict), the transition makes the movie feel a bit stretched, because the audience feels they have already seen the ending. The snowball effect would structure the two conflicts simultaneously, cutting back and forth between them and skipping the low point of transiting from one to the next.
An example of using this effect would be the final act of The Dark Knight. First we see Gordon talking to Batman, then Gordon gets Harvey’s call, Batman fights Joker’s thugs and then the final Harvey and Batman moments. If it was written normally, we’d have Gordon talk to Batman > Batman fights the Joker’s thugs> Gordon gets Harvey’s call> Gordon talks to Harvey and then Batman comes in. But to have more tension and action in the ending, we get to see the snowball effect in use. When Batman is fighting the thugs, we also think about what might happen to Gordon.
Nolan is one of the directors using it today in movies. He likes it so much that the last 80 minutes of Inception and the whole of Dunkirk are told using the snowball effect. Dunkirk has three different timelines, with each one having its own storyline, which converge over one-another at the end. In fact, by the time the ending comes, they have overlapped each other.
When Should You Use It
The snowball effect helps you to keep the audience engaged, and if you feel that your final act is a bit stretched, try using it.