The theory behind a facepulling headgear is rather simple. You want to apply constant forces on the palate to move it forward, upward and to expand it. Creating a headgear that is light enough to be worn for hours and that is comfortable enough to sleep with is the challenge.
First, there is the forward force. The biggest surface to apply this force is the anterior region of the palate, from where it will drag the maxilla forward. A forward force alone is problematic, because it would rotate the maxilla in a counter-clockwise direction and disrupt a healthy bite.