The movie "Gattaca" help?

Before Vincent left home to go out into the world of work, he had one last swimming completion against his brother (Anton)-- not to be confused with the last swim at the end of the film. Vincent stated: "It was the one moment in our lives when my brother was not as strong as he believed-- It was the moment that made everything else possible." What happened during the swimming event, and what dud Vincent mean by " it made everything else possible"?