This slightly slackens the inferior fibres of the glenohumeral ligament, allowing an abduction of 90°. Combining abduction with 30° flexion in the plane of the scapula causes a delay in the tightening of the ligament resulting in a maximum abduction of 110°.
The text describes how altering the movement of the shoulder joint (abduction and flexion) affects the tension of the glenohumeral ligament.  It explains that relaxing the ligament slightly allows for greater abduction before it tightens, leading to a maximum abduction angle of 110°.