A pro-baseball pitcher sends a curve ball to his opponent while many fans relax and enjoy the game.
A pitcher is playing baseball
A pro-baseball pitcher is also a pitcher and his sending a curve ball is part of the description of playing baseball.
A pro-baseball pitcher throws a curve ball, and this action is usually toward an opponent.
The pitcher is mentioned in both sentences. The first sentence states the pitcher is pro-baseball and sends a curve ball, so it's implied he is playing baseball.