Studying the Masticatory Forces and the biomechanics of the mandible, in 2010 Dr. Ali Nankali, expressed the theorem of the movement of the mandible at the time of Mastication as follow.
In general there are three steps:
The First step is when mandible starts to move closer to the Maxillary bone. At this time the two teeth arcs become closer and press the food.
The second step is when the mandible moves internally. This movement happens with angulations which simultaneously move the lower teeth arc downwards. This act happens mainly because of two reasons, the morphology of the involved teeth (cusps) and the anatomy of TMJ.
The third step is when mandible returns to its initial position.
This path is a curve.