Streamline after a start or a turn

The swimmer is moving the fastest as they go through the air and into the water off of a start, and as they push off of the wall after a turn. By streamlining they try to maintain some of that "faster than swimming" speed as long as they can. Some will do some dolphin or "fish" kicks while still underwater. Everyone must surface before they have traveled 15-meters (except breaststrokers, they do something different, called a pullout, that includes a larger than usual pull).