Some kids seem to think this is my way of catching them not paying attention. It can be, but mostly it's not. One of the best tricks to check understanding is to ask students to explain directions, a concept or the plot of a text in their own words. Why? Because being able to translate it into their words means that they have to process the information in order to complete the task.
The problem is that sometimes students will repeat what I've said word for word. This is memorization and it does NOT mean the that the information has been absorbed. Computers can pull up random information. People take the random information, filter into their own words and then use it to complete a task.
Because of the emphasis on standardized testing, many students are falling into the trap of memorization. If I summarize a text for them, then ask them to summarize the text on a test, they will sometimes write down exactly what I said. It's almost shocking how much of my speech they remember exactly. The problem is that if I ask them to relate it to something in real life, they just can't. Not only that, but they get hung up on what is the "right" answer. Anyone with an English teach who is worth anything will tell you that in English, there often is not "right" answer. This helps students break out of the testing box and think for themselves.
Another reason why asking someone to summarize what you've said is a good idea is to make sure your directions are understandable. Sometimes things make sense to teachers because, well, we made them. Students don't always understand overly long and complicated directions. Asking a student to summarize the directions is a good way to see if the directions are even going to be followed. If no one can summarize the directions, you need to explain it differently and then see if someone can summarize them back to you.
Students speak their own way, which is different than the way teachers speak. Tests even have their own language that students sometimes don't understand. If a student is to be tested on a key concept, it's important for them to be able to understand the testing language over that concept. Better yet, we should probably revise tests so they are less about testing language and more about student comprehension.