The sensors are mounted in a sleep mask worn over your eyes (nothing touches your eyelids).
When the NovaDreamer®'s microprocessor receives the right signals from the sensors and decides that you are dreaming, it delivers a cue to remind you to recognize that you're dreaming, that is, to become lucid. The cue to become lucid is a light or sound from the mask, in a variety of patterns, according to your choice. It is just the right brightness or volume to enter your dream without awakening you.
This is similar to the alarm clock coming into your dream in the morning. Having prepared to recognize the cue by following the accompanying exercises, when the cue comes you realize it is telling you that you're dreaming, and with your lucidity you are free to embark on unlimited adventure.