An experience gets memorized based on its emotional strength, so, for example, a strong negative experience (a trauma) will get memorized quite well. Later on, in a sort-of realed situation one gets to recall and relive such stored experience - the brain sort of "believes" it's important to remember. If the re-living is also strongly emotional, the memory gets "hardened", but if the reliving is less emotional the memory gets "softened". As you go through life, most negative experiences gets softer and softer with each recall until each one disappears altogether for lack of emotional response, and thus subjective importance. However a small number of such memories become immortal - each re-living of one such memory is as emotionally strong as the previous one, and so the memory of the pain never weakens. Such stored memory is called "Sarkana". The problem with carrying around too many Sarkanas is that eventually any event in your life brings back a negative memory from the past, so you spend your life reacting to something that happened long time ago and bears only superficial resemblance to the present situation.
The immediate purpose of the Viapssana meditation is to learn the ability to direct your attention where you want it to go, not where the pain pulls you, to learn dispassionate observation of your own suffering. Bi learning to ignore an itch on your nose, you also learn to ignore an itch in your soul. The less emotional response you generate, the softer the memory becomes, until it evaporates altogether. When completely through (a multi-year process), you only react to the present situation, not to any ghosts of the past.
You can make a surprising amount of progress in 10 days, but you have to keep at it to maintain the gains. I found it impossibly hard to keep meditating on my own.