I don't personally have a dog but I taught a roommate's dog to drop the ball by dropping a piece of chicken breast when he was looking up with the ball in his mouth and saying "drop it or drobbit!" as it descended. Then he dropped the ball and learned its more fun to drop it!
One knows paw. The other knows sit. They do it if they feel like it. So it's more about if they want to indulge us or not. They will return to whistle which is useful