Sentence Correction

From Manhattan SC

On the GMAT , it is sometimes preferable to insert a modifier using a relative pronoun and a simple verb tense than using an ing form of a verb. There were a few sentences that used ing form, and I thought they sounded right, but they weren't!

Now how does one decide that the above is preferable, cant rely on my ears for awkwardness of a sentence as it didnt get me to the right answer.

Gmatrant, Please post examples that way we can get a better picture an explain.