This stuff is hard to explain textually as it's more about feelings you have, but if I were to try.. during meditation bad memories will enter your focus and set off a cascade of negative reactions. But the deeper into the meditation you are the harder it is to shake your focus. Instead you end up observing these patterns of thoughts and how they evoke one another. Your let them have their moment in the sun and then they fade into the background as they failed to provoke you. Some stuff I've read suggests that the brain also may rewire these memories to permanently have less of an emotional impact. After many sessions you literally have to go out of your way to bring these memories back up!