With this method, you complete as many technically correct reps as you can in the 4-6 rep range. Your goal will then be to double that number of reps. To do that you take short rest periods (rest-pauses).
For example, let's say that you get 5 reps in the initial bout. This means you want to do 10 total reps for your set. After your 5 reps, you'll rest for 15 seconds. Then you might get an extra 3 reps. This means you'll need to get 2 more reps. Rest another 15 seconds and then you'll be capable of getting the last 2 reps.