I've heard different advice from many different sources, so I tried to ride the line. I mainly just gave a full-sentence resume-style description of what I did, but usually added a sentence at the end about what I learned from the experience or why I valued doing it, or an introductory short sentence to give the experience some context if it was a bit out of left field.
I doubt there's really a right answer to which way to do it. Probably each school/adcomm has a different take on what they expect/prefer. If you're really concerned, I guess you could contact your top choice school and see if you can get an answer from someone there.