Intensity is the rate at which the cells draw on their batteries. As intensity increases the time at which it can be maintained decreases. High intensity activities have to be followed by periods of lower intensity so the cells can recover. You might notice that if you have to multitask continuously at work, you feel tired when you get home. The intensity in that case was high when you were at work, which is why it had to become lower so it could recover. The same is true if you were to exercise, at some point you would have to rest.
The ability to alternate between high and low intensities is the key to all learning. New learning starts at high intensity since it takes a lot of intensity for you to be extra perceptive. That is why you remember something like a near car accident in slow motion. Learning is completed at lower intensities. That is when you sort out what happened. If you could only function at high intensity, you would always be overwhelmed and never make sense of all the things circulating in your head. If you could only function at low intensity there would be nothing to sort out and you would be bored and unable to grow.
The same could be said for all your systems. For example at higher intensities your more urgent immune system is active. The longer term immune system is more active during lower intensity. At high intensity, you are somewhat protected from infection and injury because the high intensity your systems are very protective. On the other hand the actual act of repair occurs at low intensity.
Intensity of Anticipation Versus Activity
The skill behind every skill has to do with how much intensity is required. If you had to lift 200 lbs, two things would happen. First, you would sort of get worked up in anticipation of having to lift it, then you would actually lift it. The two parts on intensity involve a level of anticipation, as well as a level of activity. If you are going to travel 10 miles, it would be a mistake to get all worked up. You have to stay relaxed and pace yourself. The actual activity draws on the cells battery while the degree of anticipation determines if fuel needs to be driven into the cell ahead of time.
Low intensity training requires mild activity like walking or jogging. Mild activity draws on the battery slowly, which means the slower burning food is adequate to fuel the cell. The use of the slow burning fuel creates a deficit of slow burning fuel. Because of the deficit created, the body increases the capacity of the slow burning fuel during sleep so that the next time the same activity can be performed with less strain.
High intensity training requires an activity like sprinting or weight lifting when the cell rapidly draw on its batteries. In order keep the batteries charged, nutrition must also be delivered to the cell. Because this happens so quickly, nutrition is driven into the cell. Strain could occur in the muscle or brain related to the activity, or in nutrition system. During a low intensity recovery period, those areas strained become more competent, and the ability to do a high intensity activity improves.
The low intensity and high intensity systems both improve based a recovery period after being strained. The strain is created by the activity directly as well as the deficit in the nutrition system. This strain can only occur if both the nutrition system and the activity are at a similar intensity. Whenever the nutrition system is at high intensity and the activity is at low intensity, the body adjusts in the form of a lower capacity which is the opposite of training. Conversely if the activity was at high intensity and the nutrition delivery was at low intensity, the system could be irreversibly damaged . This is all covered in more detail in the article on deconditioning.