Describe a situation when you would need to adjust the rate you are reading.

You could possibly need to truly need to comprehend every single sentence in the book, which would cause you to read at a slower rate.

Another reason would be if you are challenging yourself to read a certain amount of books within a time table, then you might have to increase your reading rate.

Possibly in a test you might need to read at a faster rate if you are running out of time.