Usage Rules for "dash" with respect to GMAT SC

Dear kinjiGC,

First of all, here's are some thoughts from Lucas, the "punctuation guru" at Magoosh:
When we say "dash," we might be referring to a couple of different punctuation marks, really. You're probably referring to the em dash. There are many uses for the em dash in more informal contexts. Basically, they can replace other punctuation marks such as commas, colons, semicolons, and parentheses while adding extra emphasis. But again, that's informally. On the GMAT, you probably won't see many em dashes, and a sentence correction question wouldn't hinge on their usage. If you did see an em dash on GMAT sentence corrections, it would be used to mark an aside, similar to parentheses. If the aside was placed within a sentence, then there would be two em dashes surrounding the break in thought.

By the way, if you're using em dashes in your essay, use them sparingly. They can almost always be replaced by other punctuation marks, and in formal writing it's usually best to do just that. If you do use an em dash, follow the same rules of the other punctuation mark that it is replacing. For example, "The problem is—" would be incorrect, just as "The problem is:" would be incorrect, since a colon used in this way should follow a complete independent clause.

I will just echo what my brilliant colleague says here. To the best of my knowledge, the em dash appears only in a few SC questions in the OG, for example #97, #98, #102, and #132. In most cases, a pair of em dashes are used to set off a parenthetical remark, something added for extra description or clarity. In no cases does any question test the dashes in an underlined sections. From the looks of the examples in the OG, the GMAT will never test the use of the dashes direct. They are much more likely to include dashes in a part of the sentence not underlined, simply to provide more information in case you were curious.

Does all this make sense?
Mike