I would assume that the effects of caffeine have to do with decreasing alpha brain waves and increasing beta brain waves. Getting into state (whether using PhotoReading, Lasonov's techniques, Wenger's Image Streaming, etc.) tends to increase alpha wave production, which is related to relaxed states of mind and body. It's these relaxed states that have been found conducive to accelerated learning. I would also guess that there is an effect due to constriction of blood vessels that would lessen the amount of oxygen getting to the brain, which might impede optimal functioning of the mind during the learning process (one of the things suggested in the book, the home course, and the seminar is to drink pleanty of water - it helps as a transport mechanism for oxygen).
