Final answer:
The initial movement of the mandible during mouth opening is a downward hinging, followed by an anterior gliding motion of the articular disc and mandible. The specific amount of mouth opening produced by this initial hinging movement is not detailed in the reference material.
Step-by-step explanation:
The initial movement of the mandible during the opening of the mouth is a downward hinging of the mandible. This movement is immediately followed by an anterior gliding motion of the articular disc and mandible. Together, these movements allow the mouth to open. Specifically, the downward hinging of the mandible is the first phase that contributes to the initial mouth opening. However, the extent of mouth opening this action produces is not specified in the question or the provided reference material. To understand the full mechanism and range of mouth opening, further study of the temporomandibular joint (TMJ) is suggested.