I had the same question for my instructor! And we both laughed. The answer is: release the nose or mouth seal to breath, then close it again, holding the breath.

Most importantly, the entire exercise has to be done with great serenity and relaxation. Notice how when you are very relaxed and concentrated, your breath can slow and even approach stopping. This is the ideal state for using this type of exercise. Some meditators experience extended pauses on their breath (it can even stop for quite awhile), especially when entering samadhi. It is at this stage that this type of practice is most effective, and easy. Of course, this depends upon having already developed the ability to reach that stage.