The story
Possibly an allusion to painting, in which a painter typically finishes a work with short, careful, finishing strokes of the paint brush, or an allusion to golf, in which a player concludes each hole by making short strokes with a putter. In swimming competition, as a swimmer nearly reaches a wall to turn or to finish, the competitor might take a shorter stroke to start the turn smoothly, or to finish faster.
Etymology from Wiktionary, CC BY-SA 4.0.