You split the run into segments. A segment is usually starts when load the save file and ends when you exit the save file, or basically one continuous section of the run. For the AC games, you would make a backup save file then start running a segment. When you fail a segment or do it poorly, you can load the backup file, then try again. Rinse and repeat until you're satisfied with you fast you went in that segment, update the backup save, and continue to the next segment of the run. After you complete the game, you stitch all the segments into one video, completing your segmenting run. One would do this to show how fast one could beat the game as humanly possible. It usually is a lot of grinding and replaying the same segment as you typically go for the craziest and most optimal strats in the game found at the time. Hope that helps :)

I'm making an optimal console segmented run but only the week-end when I can play on the PS3. Ectortutu's any% PC segmented run isn't optimal so if you want to try making it, go for it ! We are here to present you everything that we found. ACII Segmented console runs won't be possible actually because we can't make backup saves...