To my knowledge Spirit Walking on the Astral Plane only happens during a Coup de Grace where you actually loose spirit in the process. A normal death (unconscious) will result in no spirit loss or forced walk, you just wake up after a brief period and continue your play.
When you die (Coup de Grace) you are transported to the spirit world, a separate location than the physical world. From the drop point in the Astral Plane you must locate your corpse and rejoin your body. Family members such as wife or siblings make this much easier as your tie to the physical world is much stronger. From my understanding you follow a silverish cord/rope back to your corpse. When you locate your corpse you drop near your corpse so you can inhabit it. As you grow older and have less spirit overall, this cord becomes fainter and harder to follow till such a time when your spirit is so low that you can't see the cord anymore and are unable to locate your corpse resulting in permadeath.
