Headgear takes on many forms, but they all have in common the application of orthopedic (heavy) force.
The most common uses the neck as the anchor; i find that it extrudes the molars as well as moving rearwards. This causes the TMJ to come out of its' socket... Besides being inefficient (heavy intermittent force) they are just not worn as directed in most cases. Other means of maxillary restraint!