a swimming stroke that resembles the crawl except the swimmer lies on his or her back
<noun.act> [ verb ]
swim on one's back
<verb.motion>
backstroke \back"stroke`\ n. 1. a swimming stroke that resembles the crawl except the swimmer lies on his or her back. It is usually executed with backward-moving circular arm strokes and a flutter kick. [WordNet 1.5]
Berkoff and Mitchell go about their business differently, but both report they got into the backstroke the same way.