What is the author’s central message about teamwork in Chapters 19 and 20?

The themes of teamwork and vulnerability are important in this section. While the Washington boys have been rowing poorly, their poor performance forces them to be honest with one another about their fears and anxieties in a way that they haven't been before. This allows them to support one another in new ways and rise to the occasion. Just as Pocock told Joe earlier in the season, the boat needs every boy to trust one another completely and give themselves completely to the boat. That's the only way that they will be able to consistently find their swing. It rings true again as Washington gets their rhythm back.