Direction of force by class III elastic in face mask?
15-20 degree downward from occlusion
15-20 degree upward from occlusion
35-40 deg downward from occlusion
5-10 deg downword from occlusion
Biomechanical Considerations
The maxilla can be advanced 2-4 mm forward over a period of 8-12 months. The amount of maxillary movement is influenced by a number of factors like:

Amount of force: Successful maxillary protraction can be brought about by 300-500 gm of force per side in the primary or mixed dentition.
Direction of force: According to most authors, a 15- 20° downward pull to the occlusal plane is required to produce forward maxillary movement.
	In most cases of maxillary deficiency, maxilla is deficient in the vertical plane as well, therefore, a slight downward, direction of force is usually desirable. The line of force passes below the center of resistance of the maxilla producing a counter-clockwise moment on the maxilla and dentition. This results in a possible extrusion of maxillary posterior teeth leading to a downward and backward rotation of the mandible.
	However, in patients with increased anterior facial height, downward pull is contradicted.
Duration of force: A review of literature shows duration to vary between 3 and 16 months. On an average at least 8-12 months of wear is required to produce the desired effect.
Frequency of use: 12-14 hrs/day.

Key Concept:
According to most authors, a 15- 20° downward pull to the occlusal plane is required to produce forward maxillary movement.
Reference: Textbook of orthodontics 2nd ed Gurkeerat Singh page no 502