When people speak or sing, their vocal cords vibrate. When a man sings falsetto only the edges of his vocal cords vibrate. These produce harmonics. It is similar to playing harmonics on a string instrument by lightly touching the string at a certain point so that only part of the string vibrates. When a man sings falsetto his is usually one octave higher than his normal singing voice. When people sing in falsetto, their voice can be breathy sometimes.