Final answer:
The motion that unlocks the subtalar joint for inversion and eversion is the eversion of the foot. The subtalar joint allows gliding movements between the talus and calcaneus bones, enabling the foot to adapt to various surfaces.
Step-by-step explanation:
The subtalar joint is an articulation between the talus and the calcaneus bones of the foot, which allows for the motions of inversion and eversion. These movements are important for walking and balancing the body. The motion that unlocks the subtalar joint for it to perform inversion or eversion is primarily the eversion of the foot, involving the intertarsal joints where the bottom of the foot is turned laterally, away from the midline.
Additional ligaments support the joint and resist excess eversion or inversion. This complexity allows the foot to adapt to different types of surfaces and to assist in shock absorption when walking or running.