This is a great meditation technique. I've heard a couple different varieties of this.

In one, you review the day's events in your mind as you are lying down for sleep at night. You go through sequentially, in reverse, letting go of each event as you think about it. I'm not sure, but I think this is described in Rinpoche's Dream Yoga book. I know I used to do it when I practiced Dream Yoga. "Letting go" helps break the karmic cycle.

The Monroe Institute has something similar called the Energy Conversion Box. Any thought that is bothering you or occupying your thoughts goes in this imaginary strong box with a heavy lid. Once you dump everything in there and close the lid, you turn the device on, and it coverts all the negativity and stress into positive energy.

These are just different ways of visualizing the same basic concept. All the unimportant, but stressful, crap we worry about on a daily basis can stick with us. Identifying it and letting it go can help free up our attention for more important things.