One of the books I had to read for my senior year English class was Fifth Business by Robertson Davies. The story started with a snowball to the face -- literally. Around the same time, I was learning about writing and how to captivate a reader in the beginning of the novel. How appropriate, I thought at the time, a snowball to the face. That image has stuck with me.
You hear about needing "a hook" to snag your reader, a phrase, an image or an instant premise on which the reader's interest quickly becomes engaged, leading them to keep reading your book.
That hook can be a snippet of dialogue that needs to be explained or a startling image that requires a little more thought. In short, it's something that captures your attention and, hopefully, your curiousity.
The snowball that goes awry and hits an innocent woman is such an event. Even more importantly, the incident of the mistargeted snowball sets into play the entire plot of the book. A hook, if used properly, should never be intended simply for shock value. It should seamlessly tie into the rest of your story and perhaps even be relevant at the very end, as the snowball is for Dunstable Ramsay and the Dempster family.
To extend the fishing analogy, the hook is not just there to dangle and look pretty; it's there to land a fish (or a reader, in our case). Nothing is worse than the hapless reader turning to that first page after finishing the book and wondering why that curious little incident that first piqued his interest was never explained in the end. It's a hook, not a mystery.
Start your story with action or words that startle and intrigue. Start with a bang. Start with a snowball to the reader's face.