So far i'm loving JUDD, to explain it properly i'll copy and paste the official bumpf
Every day on the Johnson UpDayDownDay Diet™ is either an Up Day or a Down Day. Since you are free to eat as much as you would like to on Up Days (without intentionally overeating), the Down Days are when you will do most of the "heavy lifting" on the diet. By restricting calories on Down Days, you will lose weight and you'll also activate SIRT1, a gene that has been shown to prevent disease, improve metabolism, and extend lifespan. Dr. James Johnson wrote The Alternate-Day Diet to help individuals lose weight easily and effectively. The book includes recipes, menus, and specific information and tips for Down Day success.
The Induction Phase – 2 Weeks
During the first two weeks of the diet, your Down Day calorie intake will consist of no more than 20 percent of what you would normally consume. A calorie counter is provided below so that you can figure out what your normal calorie intake and Down Day calorie intake should be. However, to simplify matters, Dr. Johnson has rounded off daily intake to 500 calories for Down Days during the induction phase.