Why might you suddenly exhale when trying to fall asleep like a reverse hiccup?

You are likely referring to the phenomenon known as a "hypnic jerk," or sleep start. It is a sudden involuntary muscle contraction that occurs as a person is falling asleep. It is characterized by a brief, jerking movement of the body and is often accompanied by a feeling of falling or being startled. Hypnic jerks are harmless and usually last only a few seconds, but they can be disturbing and disruptive to sleep. They are thought to be caused by a temporary glitch in the brain's sleep-wake cycle, specifically in the brainstem, which controls muscle movements and the transition from wakefulness to sleep. Factors such as stress, anxiety, fatigue, and certain medications may increase the likelihood of experiencing hypnic jerks.