A race car has gauges and sensors to help monitor the engine and it also has oil. Without oil, the engine would grind to a halt, destroying itself in the process. The oil is everywhere, keeping the parts of the engine working smoothly, cooling them and ensuring they perform well under stress.
The ScrumMaster is like that. He has embedded sensors and gauges that allow him to identify when the team is not performing to its ability and he has the skills (lubrication) needed to assist in correcting the issues. Being the ScrumMaster is a hard job. A good ScrumMaster is someone who can read non-verbal communication, is comfortable with conflict, is an effective communicator, can build trust and earn respect, and understands team dynamics. A good ScrumMaster can evolution-ize development. He can build trust not only in the team but with customers as well.