I think of pacing in my writing somewhat similar to tempo in music. I used to play the flute, clarinet and piano. While playing the piano, I learned about tempo and depending on the music, you would slow down, speed up, play softly or loud, hold a note long or short, that creates emotion and feeling into the song being played.
Well, I feel doing this with writing is not that far from the same if used as a technique. If you have a lot of action going on, you shorten your sentences and are more quick to the point. In scenes where you are layering details and building up characterization, these tend to be more detailed and visual. So there's a bit more written in the sentence.
So, depending on your story, you can add tempo to your writing effectively.
Here are some helpful articles on how to use pacing in your novel: