Many people think the writing section of the SAT is only the essay, but that couldn't be further from the truth. The essay is small part of the writing section. Recognizing and correcting misplaced prepositional phrases is one of the skills tested on the SAT writing section.
A prepositional phrase is a group of words starting with a preposition and ending with the object of the preposition, usually a noun. Examples include: to the store; between the cars; into the pool etc. These phrases usually modify something else in the sentence. And the phrase needs to be next to the word/noun that it modifies.
There is a famous Groucho Marx joke that goes something like this: "I shot an elephant in my pajamas…. How the elephant got in my pajamas, I will never know." In this example "IN MY PAJAMAS" is the prepositional phrase that is supposed to be modifying the subject of the sentence I. Of course since Groucho Marx laughed at his error and corrected it, we know that he knew it was incorrect.
Tonight on the news: "A 65 year old woman boarding a plane with a gun." Uh??? What was the plane doing with the gun??? YUP! That's incorrect. WITH A GUN is a prepositional phrase that is telling more about, or modifying, the WOMAN and so should be attached to that subject. It should read "A 65 year old woman with a gun was boarding a plane." Once you get used to writing/saying it the right way, the wrong way is actually funny.