Obviously, it is important to select a body signal which can be accessed from within a dream, such as dream eye movements leading to movements of the real eyes, dream body muscle activity causing measurable activity of the real muscles, dream respiration affecting the real respiration, or selected tasks leading to specific brain activity (compare chapter 2).
Another important point is that the body signaling must not lead to waking up the sleeper. For example, using changes of respiration speed as a signaling method is not very practical as it directly influences the oxygenation level of the blood and thus might lead to waking up the sleeper.
Additionally, the mode of transportation has to be detectable from the wake world without affecting the sleep of the dreamer. For example, eye movements can be recorded by EOG, muscle movements by EMG, respiration by nasal airflow recordings and specific brain activation by EEG, functional Near-Infrared Spectroscopy (fNIRS), functional Magnetic Resonance Imaging (fMRI), or other techniques.