I always tell the players to keep their neck muscles tight or firm them up so you don't strain their neck. The main point is that you always want to attack the ball and not let it attack you. You don't want the ball to hit you. You want to hit the ball. You have to be somewhat aggressive.

It forces the player not to use their legs. In heading, you don't really generate power from the legs, you generate it from the waist up. You could also build to having them do diving headers with the second exercise.

After I do those three, then I will do about 30 seconds where they repeat the three exercises and then back down again. It gives them repetition and a little bit of fitness included.