From the Boy's Own Book of Outdoor Sports (early 1900s)

This is employed when the swimmer wishes to raise his head as high out of the water as possible, and is particularly useful if he is reconnoitering, or if he is trying to save a drowning person, or if he wishes to grasp a bough or a rope above his head. The best method of making the stroke is as follows: Keep the body perpendicular, and make precisely the same stroke with the legs as is done in ordinary swimming. This action will keep the head freely out of the water, and if assisted by the hands the body will rise as far as the shoulders.

Some persons literally "tread" the water, striking each foot alternately as if they were ascending a staircase. We have thoroughly tried both methods, and much prefer the former.

See also: List of swimming styles