Legs Underwater
Carlos demonstrates his breaststroke as we focus on his legs from underwater.
Watch how his feet slide forward, following the legs as they're drawn up by the hips. The feet side forward, and higher than the knees during the initiation of the recovery. This keeps them from creating any additional resistance.
You'll also notice that he turns the bottoms of his feet inward, toward each other, on the finish of the kick. This gets him a good finish of the kick, and then he briefly holds the feet together to clean up the back of the bodyline.