Peroneus Brevis, like Peroneus Longus, attaches outside of the fibula. It attaches to the lower portion of the fibula between the fibular and peroneus longus.
Like the peroneus longus, the tendon of the peroneus brevis passes downwards behind the lateral malleosus, the "bump" at the side of the ankle. It passes down the outside of the calcaneus, angling forwards and downards so that it passes in front of the trochlear process.
The trochlear process is a squarish shaped projection located on the outside surface of the heel bone or calcaneus.
Where the lower tendon of the peroneus longus passes behind the trochlear process and from there proceeds to wrap beneath the foot at the cuboid bone, the peroneus brevis passes along the outside of the cuboid and then attaches to the outer edge of the outermost metatarsal.
The outermost metatarsal forms the first isolated bone of the little toe.
Where the peroneus longus may create an upwards and rearwards pull on the cuboid, assuming a standing position, the peroneus brevis creates a rearwards pull on the outermost metatarsal. Both muscles acting together can help reinforce the arch of the other foot.
Assuming an active tibialis posterior, an active peroneus longus and peroneus brevis can work together to help stabilize the outer edge of the foot. In turn they can create a downwards pull on the fibula, helping to anchor it. Since the biceps femoris attaches to the top end of the fibula, the above mentioned co-activation of the peroneus brevis can help to provide a stable anchor for the biceps femoris.
With the foot unweighted, say while sitting in staff pose with the feet straight out in front, activation of the peroneus brevis may help to pull the outer edges of the feet rearwards, towards the hips.
In a pose like pigeon pose, with the front leg hip on the ground, activation of the peroneus brevis may help to stiffen the outer edge of the foot making it easier to "Press" the outer edge of the foot into the ground.