When someone talks about the whistle register, they are referring to a specific range that applies only to women. Hitting the whistle register will allow you to increase your high range and expand your total overall range of pitches. In men, a similar register exists called falsetto. The whistle register will vary slightly depending on your physical makeup, but for most women the register begins on the B two spaces above the treble clef staff.
Stand erect, or sit with your back straight and your shoulders relaxed. Release any tension in your neck, throat, shoulders and chest.
Breathe from your diaphragm by using the diaphragm muscle, expanding it to pull air into your lungs. Control your airflow as you sing in the whistle register by slowly pushing air out of your lungs using these muscles.
Relax your lips, jaw and tongue and all the muscles in your face. Open the back of your throat by maintaining proper posture and holding your head up high.
Start on an A-sharp above the staff and sing down a perfect fifth without entering the whistle register.
Sing a B above the staff and feel the change in your throat as you enter the whistle register. Direct your air flow so that you feel only the outer edges of the vocal cords vibrating. Exhaling too much air will prevent you from singing in the whistle register, since too much air will move the entire vocal cord.
Stop singing if you feel any discomfort or pain. Singing in the whistle register should not be painful. Consider finding a qualified vocal instructor to help you sing in the whistle register.