This metaphor is useful for car enthusiasts, or anyone really, as long as they understand the concept of a clutch in a manual transmission vehicle.
It's as if you're driving a car on your journey through life. However, you begin to notice over time that the engine in this car is very finicky, almost like it has a mind of its own. It occasionally revs up very high, or bogs down low. As it does this, the car speeds up or slows to a crawl, regardless of the speed limit, upcoming turns, or what the traffic is like around you. Naturally, this can make it difficult for you to effectively drive in the direction you want to go. Your initial response is to try to control the pace of the engine by hitting the brakes when it speeds up or hitting the gas when it slows down. You push down hard on the brake, and you slam the gas pedal in response to the engine acting up. However, you notice over time that this does not work very well or for very long, and the engine revs even higher or bogs down even lower when you try to fight it. What's more is while you have been putting all of your energy into those pedals, your attention is no longer on the road or the direction you are headed. What if there were an alternative to the gas and brake pedals. What if you looked down to find that there was a clutch, hidden over in a dark area of the floorboard? Instead of trying to control this finicky, unpredictable engine, you could simply take the car out of gear. This would eliminate the need to control or change the pace of the engine, and instead as it begins to race or bog down, you could simply push in the clutch. When the pace of the engine is appropriate or effective for the road that you are on, you could let the clutch back out. The engine no longer controls the speed of the car. You do. And you are now free to concentrate on the road ahead of you. We can think of our minds as being like the engine in this example, and willingness, acceptance, and mindful awareness are all like the clutch. When our mind begins racing, telling us to run away or escape, showing us images of possible dangerous scenarios, reminding us of painful past events, or bogging us down with evaluations, judgments, and negative predictions, we can choose to engage the clutch and willingly observe these things without allowing our behavior (the car itself) to automatically speed up or slow down. When we find that our mind is being helpful, we can release the clutch and allow it to do its work. We can't ever successfully gain control of the engine, but we can gain control of the speed and direction of the car.