I'll admit, this is one of my favorite things about the Prio.
The huarache-sandal inspired heel strap can be used to hold your heel and ankle in place. Pull the straps through the ankle holes in the sole, and then lock them in place with the laces.
The instep straps can be used to secure your instep, which can free up your toes and forefoot to relax and function better. Use the laces to tighten or loosen them to your liking.