What are the rules on the use of dashes?  The Greggs Manual says you can use them in place of the comma, the semicolon, the colon, or parentheses when you require special emphasis.  Do you think the sentence shown below is correct in the use of dashes?

Chanting is an easy tool because you can chant anywhere—out loud or silently—and change your state of being.

Thanks,

gmk

I've seen some writing where the writer used A LOT of dashes. I guess they were Mdashes. At first it seemed pretty cool, kind of stylish, but after 5 or 6 on a page, I thought it was too much. It started to get really repetitive, and I started counting dashes instead of paying attention to the story.