Meanwhile, use a boning knife
to cut a slit in each chicken breast along one long side. Cut through the center of the chicken, with the knife parallel to the cutting board, leaving the opposite side attached. You're trying to create a deep pocket in each breast, with a 1/2-inch border around the three attached sides. (See post images for clarification.) Cut evenly, as to not leave thick uncut sections that won't cook through in the oven. Then sprinkle each breast on all sides with cumin, chili powder, and salt.