The tonic category of muscles in the upper body includes the neck extensors, pectoral muscles, upper trapezius, levator scapulae etc. These muscles pull your head forward, shoulders forward and up, arms in, and flex the wrists and fingers. The Brugger exercise uses the antagonistic muscles, those that extend the thumb, fingers, wrist, and extend and externally rotate the shoulders.
Think of the two categories of muscles as generating forces that should work harmoniously to allow for a balanced posture just as the bass and treble knobs on a radio should be balanced to create harmonious sounds. Turn up the bass too high and the sound is no longer harmonious as the treble gets drowned out. Likewise if the treble is turned up too high or the bass too low.
By "turning up" the activity in the phasic group of muscles we can, through reciprocal inhibition "turn down" activity in the tonic group of muscles allowing for further stretching and a more balanced posture.
To achieve the starting position:
Perform this exercise slowly taking a full 4 seconds to bring your hands back, hold for 2 seconds and take a full 4 seconds to come back to the starting position with your elbows bend, hands in front of you. Repeat this for 2 minutes or until you feel you are no longer able to complete the repetitions smoothly as instructed.