"The complicated mechanism" that people are refering to, might be that of the sensory distortion. Psychedelics tend to not just affect the receptors and the release of neurotransmitters, but also affects the signal in other ways, unusual links between centers in the brain, extreme distortion of the electromagnetic neurosignals and stuff like that. A good theoretical (but complex) explanation can be found in a book called Psychedelic Information Theory, which is an awesome introduction to the neurobiology behind the psychedelic experience. http://www.tripzine.com/listing.php?id=pit_toc here's a link.