When taking the test you can, at any time, hit the "start break" button and pause the exam. So you can do a couple of passages and break out of the exam and it won't count more than once towards one of your 20 times.
Like another poster wrote, the annoying thing is that there is no way (that I know of) to jump back to a certain question without having to click the next key. So if you left off on question 100 last night then today you would have to sit there and hit the next key and wait for each question to load and hit the next key again and again, until you got to where you left off.
You only really have a couple of options. Either you take it like a test without the solutions on screen or you can take it with the solutions up the whole time. What this means is that if you take it in "test mode" (without the solutions on screen) then you have to finish before seeing the answers. In other words, you can't go and check your answers against a key after each individual passage like you can with a book.
So you won't be able to take a couple of passages and pause to see how you are doing. You will have to finish the test and only then can you see the solutions.