It sound interesting, but I never have heard it before and I can't find it in the link or in Google. Is it in the book? Is there other source? (Creativity advisors are sometime too creative.)
This is also mentioned in the Coursera course "Learning How to Learn" (https://www.coursera.org/learn/learning-how-to-learn). Though I don't recall/know what their source is. "Dali Sleep Technique" turns up many hits in your favorite web browser, however.
it is in "50 Secrets of Magic Craftsmanship" a bit after the "Three rules for controlling your dreams".
Edit: Search for the sentence "In this posture, you must hold a heavy key which you will keep suspended, delicately pressed between the extremities of the thumb and forefinger of your left hand." and look at the google book result, you can read the page 36.