When the curve of any cycle is above the midline, we experience a "high" in the corresponding field (physical, emotional or intellectual). If it is the physical curve, you will feel bodily well and, for example, if you were to have an operation the body would be more resilent. If it is the emotional curve, you will be in a good mood and your contact with other people will be positive. If it is the intellectual curve, your thought processes will be sharper, concentration will be easier and you may have a lot of ideas and insights.
When the curve of any cycle is below the midline, we experience a "low" in the corresponding field. If it is a physical low, you may feel weak and listless; if it is an emotional low, you may feel miserable and out of sorts; if it is a intellectual low, thinking can be foggy and concentration hard.
When any of the curves crosses the midline, this is a critical time in the corresponding field and if two curves cross the midline at the same point, this is a very critical time. Critical in the physical field means you may feel exhausted and it is better to avoid physical effort and operations. Critical in the emotional field means you may feel irritable and sad, and emotional outbreaks are likely so it is better to avoid conflicts and arguments. Critical in the intellectual field finally means sluggish thinking and it is better to postpone activities that demand a lot of concentration.
Biorhythms can't predict or explain events, they can only suggest how we may deal with them. For example, some doctors plan operations around them or sports coaches use them in their individual training programs. But it is up to you whether you believe in them or not.