Periodization is a technique that Mike Ashley believed his success came from. Periodization is the method of going through periods, or phases, during training. These phases are aimed at different goals, whether they be bulking, cutting, and/or strength training. What Mike Ashley did was alternate strength and hypertrophy training cycles. The goal behind this was to build up strength so that he would be able to lift heavier during the hypertrophy cycles and build more muscle. Perhaps the best part about periodization is that it adds variety to your training, so that rather than doing the same old workout 52 weeks a year, it changes every 3-6 months.
So why use periodization? With the changing methods of training, it keeps the muscles guessing. Kind of like how Arnold Schwarzenegger always believed in shocking the muscles with new things. Periodization is a way to shock the muscles by doing phases with different goals. If you go from doing 5 sets of 5 reps in squats with 405 to doing 4 sets of 12, 10, 8, and 8, starting at 225 and pyramiding up, that will shock the muscles because during the strength phase they became accustomed to only doing 5-6 reps per set. This helps to eliminate plateaus in muscle growth.
How do you use periodization? It is pretty simple to figure out how to use periodization. You could do what Mike Ashley did and incorporate a strength phase and a hypertrophy phase, or you could make it a bulking phase, a cutting phase, then a strength training phase. Or you could certainly come up with your own combination specific to your goals. Just be sure that the training in each phase matches what the goal of that phase is. So, for example, during the strength phase, I should be focusing on strength, rather than the hypertrophy of 10-12 reps. During the hypertrophy phase, I should not be focusing on building strength.