Exercising for a short period of time (0 to 10 seconds) requires an immediate source of energy to remake adenosine tri phosphate or ATP.
The creatine phosphate system is the first energy system in which ATP is resynthesized and is anaerobic (does not require oxygen).
When our body senses that we are beginning to exert maximal effort, it breaks down creatine phosphate for energy to resynthesize ATP, which is used up during maximal intensity.