Usually warm, most air is trapped in a shirt or jacket and cannot escape, especially when the collar is closed.
The Diffuser, a rib structure on the collar, allows moist, warm air to escape as needed. The MicroDiffuser on the collar and the cuff has the same structure but is made of finer, precise ducts. The MicroDiffuser on the cuff additionally creates a highly comfortable band on the sleeve. The diffuser constructions allow continued ventilation during physical activity.