Scrum and motorbikes: two things that I am very passionate about. Subconsciously, I could sense a parallel between them. When I actively thought about it, the picture became clearer.
Consider the team (developers, UI designers, testers) in Scrum. Its mandate is to deliver a working increment in each iteration. It is what powers the product/project. It's similar to the engine of a motorbike. It converts stored-up energy (in the form of fuel) into motion that drives the whole. Akin to a Scrum team, it delivers usable power in each cycle of the n-stroke engine. Similar to a Scrum team that iterates over the backlog, the engine iterates over the strokes in each cycle. In the way an engine gobbles up fuel to deliver power, the team completes stories from the backlog to deliver working software.
Now, you don't need to be a gearhead to know the importance of keeping the engine well oiled. That is where the ScrumMaster fits in. He is the engine oil. The oil keeps the engine running smoothly, lubricating it, ensuring that there is no grinding against the metal. Similarly, the ScrumMaster keeps the team protected from outside interference, ensures that there are no impediments in the progress, that the team is not overloaded, that there is no grinding between the team members. He keeps the team running smoothly. Good engine oil keeps the parts well lubricated even when the engine is running at a high RPM, the temperature is high, the stress is high. Similarly, a good ScrumMaster ensures that the practices are followed even when the team is under stress (the scenario of the team being under stress is infrequent but possible when the team commits to deliver more than it can complete so as to meet an aggressive goal).
Where does the product owner fit in? I believe he is like the handlebar of the motorcycle. The product owner provides direction to the team. He leads the team to the intended goal. If the team deviates, he provides correction (during sprint reviews) so that the overall destination can be reached. The way a handlebar can be used to avoid obstacles along the path, the product owner can clarify the doubts of the team to keep them focused on the right goal.
Of course, there are a lot of other parts in a motorcycle, and many more and varied parallels can be drawn, but this is what struck me. If you keep the above in prime condition, you can assure yourself of a smooth journey. Ride hard, ride well.