It basically asks you to imagine yourself in a bus and your worst problems are also traveling with you. Personify these problems as co passengers and then on the next stop they get down and you let them go.
Generally speaking those traumatic memories are there in your subconscious and will not go untill you go through them during meditation.
I try not to make it a battle against the memory. Sometimes the memory may need to be emotionally experienced and further processed. If that happens or is necessary then I start resting my attention again on what’s happening in the moment, what I’m actually feeling and experiencing physically as well as emotionally. Often times that process creates enough space to help move through the memory a bit more easily.