The phosphocreatine (PC) system uses a substance called creatine phosphate to 'glue' or resynthesise the third phosphate molecule back on to ADP to make ATP. It is capable of reforming ATP very quickly, however, because there is only a very small amount of creatine phosphate stored in the muscles the energy supply is very limited. No oxygen is required for this system and no by products such as lactic acid are produced.
This system can only be used for immediate or very short bursts of activity such as throwing a ball or running for a bus. It is used at the start of exercise for activities which require a brief maximal effort, for example, explosive events such as the long jump, javelin or 100m and is exhausted after about 10 seconds of maximal activity.
As energy demands continue after the initial burst of activity the body has to utilise other energy systems such as the lactic acid system to continue to make more ATP.