This is like a pre pre-season phase. It is a phase that every athlete should complete and probably the one that is overlooked more often than not. Of all the phases, it is probably both the easiest and the one that makes you feel the best, so you would think it would be popular. Its drawbacks are that it's somewhat boring and you don't see a lot of gain.
What you are doing is increasing your capillarity (number of capillaries), which increases your muscle's capacity for work. In doing so you will also repair micro-traumas that have occurred in your muscles during the previous season.
This is primarily achieved with GAET (general aerobic endurance training), which consists of LSD (long slow distance) work and high-repetition resistance training.
Ideally, you will finish this phase with a healthy body that is ready to tackle more intense training.
The "size matters" phase. The aim is muscle growth. This should not be confused with muscle strength; it is simply size. You are increasing your muscle size, which in turn increases its capacity for strength. Many bodybuilders spend most of their time here; and why not, since their sport demands that they look good, but relatively little performance is necessary.
This is an 8-12 repetition phase. Runners may do a lot of interval training. It is a moderately painful stage and, in general, relatively safe. Injury risk should be low.
This is the strength phase, where you train your newly grown muscles how to work. Muscles are controlled by motor units. Untrained muscles "recruit" a low number of motor units while efficient muscles "recruit" a high number. This is where strength-to-weight ratio comes from, and is of the utmost importance in any sport where body weight is a factor.
Power athletes try to get as close to 100% muscle efficiency as possible. This is fun but it is also dangerous. Exceeding 100% puts excess stress on your joints and tendons and always causes injury. Most sports injuries happen in this phase.
Training during this phase is explosive and often plyometric - which means using forces in excess of body weight to cause a reflex reaction - like jumping off a chair onto the ground. Rep schemes are very low and stress-per-unit of exercise is very high.
This is the "no pain, no gain" phase. It hurts. What this also means is that the other phases shouldn't hurt - an important thing to realize. Many people feel that they aren't exercising properly unless they hurt. This is not only masochistic, but also wrong. There is a time when you need to push through pain - and it's during this phase.
The reason it hurts is that you are working in the Creatine Phosphate and Glycolytic Cycles, and you will generate a lot of waste product called lactic acid. Lactic acid build-up is both debilitating and painful, but the aim of this cycle is to increase your anaerobic threshold, and staving off lactic acid is your goal.
The majority of sports are highly dependent upon your body's ability to perform in the CP/Glycolytic Cycle.
During this phase your rep schemes will always go until failure--more pain, yeah!
Rest is just as - if not more - important than any other phase. Without proper rest (during all the phases, not just the rest phase) all the work you do will be for naught. You will over-train, and instead of making gains, your fitness will decline.
There are two types of rest phases. One is more like a break. I think all athletes should take a month or two off a year where they don't train and let themselves recuperate, not just physically but mentally.
The other is a peaking rest phase. During this phase you will do maintenance work to keep your strength gains while allowing your micro-traumas - that have occurred through training - to recover. You will try and complete the concentric part of an exercise and eliminate the eccentric (no plyometrics) part as much as possible. The reason is that you allow your Type IIB (emergency) muscle fibers to regenerate, which can give you a 10% performance increase on the day of a big event.