This method has a fancy name but there is meaning behind it. However traumatic life has been, everybody has a few memories of pleasure, even if the pleasure did not last long. A time when their life was joyful, when their life had joy.
To use this method, you need to think about these joyful times and have a few of these times really well remembered, and kept ready in your brain.
So, you are having a bad day, unable to stop the negative thoughts that dominate your life. What to do? This is where you pick one of your well remembered joyful memories and start thinking about it. You may find it difficult to hold on to this memory, but keep bringing youself back to this memory, and understand that you will need to remind yourself over and over to think about that moment.
With practice, you will find that it becomes easier to hold your thoughts in a joyful time, and when you do fail, that it will be easier to bring your thoughts back to this happier moment.
Understand that this is not easy. It is hard work that requires perseverance. But persevere and you will be rewarded.