I know that your post is very old but I found it because I was looking for the same information. Then I found this video where the drill log feature is explained. If you know how it works I hope my post helps others. Cheers
Yes, I think people who feel their watch under/over counts lengths, this manual feature has additional use beyond just for kicking sets. I use it when I am in a crowded lane where I know I may get bogged down or need to pass someone, or the wall is too crowded to get a firm push off. It can also be useful for sprints, which sometimes don't get logged correctly. So manual input is sometimes very useful.