It sounds like you are describing the transition to sleep, and hypnagogic imagery. When I pay attention as I fall asleep, I find my thoughts start to develop into brief 'dreamlets'. They get more and more vivid, until I get so caught up, I lose my sense of self and fall unconscious. Pay attention to how this works, and you should have a better understanding of how all the WILD techniques out there are designed to trick yourself into doing this consciously.