This article is from the Aerobics Fitness FAQ, by email@example.com (Robles) with numerous contributions by others.
After any aerobic activity, the blood is pooled in the
extremities, and the heartrate is elevated. The purpose of
the cool-down is to bring the heartrate down to near-normal
and to get the blood circulating freely back to the heart.
Stopping abruptly could result in fainting or place undue
stress on the heart. The cool-down should also include
stretching to help relax the muscles which worked so hard
during the activity. The cool-down stretches also increase
flexibility, and might help to prevent DOMS (Delayed Onset Muscle
Soreness) although this has not been proven.