There's a church about a quarter mile from where I live. It's a small church; it probably holds 100 people or so. Like many churches in small villages in the UK, it's not very active, outside of the major holidays: Christmas, Easter, etc.
But the church has a bell. It rings once every 15 minutes, and marks the time of day every hour with the number of bells of the clock.
When sitting today, I noticed that the bell sound leaves an imprint in my mind. I hear the ring, I listen to it, then it fades away. But there's still an imprint of that sound in my mind, that my mind can hold on to, and almost play back if it wants.
This is what happens with thoughts as well. When I have a thought, it floats into my mind - either slowly, like a waft of smoke, or suddenly, like that church bell - then starts fading away. I let go of them, but my mind can call up some of these thoughts, or at least some of their imprints afterwards.
Perhaps this is how we keep repeating the same negative thoughts in our minds. Bad thoughts seem more potent than good thoughts; don't take my word for it, look within and I'm sure you'll agree. So it's more likely that they leave imprints. Over time, the mind may develop a full gamut of imprinted thoughts.
How can we let these imprints go? I don't know. Sitting, understanding that thoughts are impermanent, may help us let them go more quickly, and not hold on to their imprints. Trying to not get attached to them will certainly help. Have you ever noticed how, when you have a really anxious thought, it takes on a life of its own? If you can just let go of it, maybe it won't be so powerful.
Recognizing these imprints for what they are may help us let go of the thoughts that flow through our minds more easily. Perhaps, over time, it can also help us free ourselves from negative thoughts and habits.