Stephen Wilbers compares the concept of the long-short combo to that of a sling shot. Stretch out your first sentence and then ... Pow! Follow it with a concise, snappy one--the shorter the better. This adds punch, or emphasis, to what you're talking about.1
Here are a few examples:
Yesterday I read my copy four times, one word at a time, from front to back and from back to front, and today you found an error. So much for proofreading.2
"He'd mentioned his nonna, but for the first time she considered that he might have little ones. And a wife."3
Underneath, while you write you are a little nervous, not knowing how to get to what you really need to say and also a little afraid to get there. Relax. 4
Here's one for you to try:
Driving down Superior Street on a Saturday night, the sidewalks deserted, wind off the lake blowing snow through the pink light from the street lamps, the temperature stuck at twenty below, you know this isn't Paris, and it isn't even Minneapolis. This is a city called Duluth and it's at the top of the map.
1 Wilbers, Stephen, Mastering the Craft of Writing, pg 170.
2 Wilbers, Stephen, Mastering the Craft of Writing, pg 170.
3 Urry, Peggy, The Archer's Hollow.
4 Wilbers, Stephen, Mastering the Craft of Writing, pg 172/Goldberg, Natalie, Writing Down the Bones.