Question

How does changing sentence structure from long to short and choppy help build suspense?

Answer 1

Changing the sentence from a very long sentence into a short and choppy helps the suspense by not giving the reader a lot of information and making them really think and wonder.
For example:
"The stranger watched, a look in his eyes and this feeling spread throughout my body." 

or

"There was a man watching, his blue eyes had this look in them that made me shiver with fear. His pale face held no emotion and made him seem as if he was just a corpse standing on his own."

The second one might sound better, yes, but the second one really makes you think and really builds the suspense.
"Who is this stranger? What does he look like? What feeling spread throughout their body?"

So instead of knowing a lot about this stranger, you know little to nothing and it really makes you want to know more, and definitely build the suspense. 

Answer 2

Changing sentence structure helps build suspense by changing the rhythm of the sentences and paragraph.