NRG-500 Challenges are exclusive to Grand theft auto san andreas, and are required for 100% completion. The NRG-500 challenge takes place in the abandoned Dry Dock at Easter Basin, to start the side mission you must go next to the NRG-500 and hop on it to start the mission.
In the challenge you start with a NRG 500 motorcycle, have to drive through red checkpoints in a whatever order you want. You must take your own route however you like to get the fastest time.
Each time you pass through a check point you will gain +10 seconds. The NRG-500 challenge is deemed rather difficult to players.