I will scan the soc.religion.shamanism FAQ files again and comment further in a while, but for now will say that the descent to the underworld mentioned there can represent several things: drug guided visualization, drum/meditation guided trance within, or for me the psychic death experience or beast transformation during waxing gibbous moon with release/rebirth at full moon.
The ascent to the otherworld could represent lucid dreaming or out-of-body voyage in flight, either drum/meditation or drug facilitated or both, but for me it represents the pre-new-moon ascent to the "mountain" mystic/shamanic high, particularly my tortured ascent in September 1991. (My four other pre-new-moon highs were not that painful.)
During solar sunspot cycle high and medium years these were separated by 5.5 lunar month periods between the descent and the ascent. During the solar low or in Hinduism "cremation ground" years, notably 1996, I had some descent (including one during which I could not feel my own pulse) episodes without a later ascent return.