The reason we don't fully understand the mechanism behind them is that we don't truly understand how consciousness works. New research from the University of Queensland is shedding new light on what exactly is going on in our brain when we're knocked out by a general anesthetic – and it's much more complex that simply falling asleep.
A recent book by Kate Cole-Adams called Anesthesia: The Gift of Oblivion and the Mystery of Consciousness relates a variety of odd and unsettling stories about people who hear things while under anesthesia or experience odd forms of conscious awareness. The overwhelming thesis of the book suggests that we could be more aware of our surroundings during a general anesthetic than we realize. And more frighteningly is the implication that anesthesia fundamentally rewires our memories.
Research over the last decade is suggesting that anesthetics result in unconsciousness by disrupting the brain's ability to communicate with itself. Blocking transmissions between different areas in the cortex seems to result in our consciousness vanishing. This also seems to result in strange side effects, such as memory loss and post-anesthesia cognitive impairment.