English Grammar for the Utterly Confused - Laurie Rozakis 2003
Dashes
Chapter 10 Punctuation
Part 4 A Writers Tools
A dash (—) is not the same as a hyphen. The dash, or a pair of dashes, lets you interrupt a sentence to add emphasis with additional information. Use dashes lightly or you risk creating a breathless, overly informal style.
Use a dash to emphasize an example, a definition, or a contrast.
Two of the strongest animals in the jungle—the elephant and gorilla—are vegetarians.
Two of the strongest animals in the jungle are vegetarians—the elephant and gorilla.