On 1.8-liter NA and NB Miatas the OEM differential housing has a notch on the passenger-side arm that is designed to help the unit absorb energy in the case of an accident. Mazda engineers designed the housing in such a way that the differential would separate first during a major street accident, absorbing energy and protecting larger components from damage. This is a great idea for a road car, but in a race car, especially a Spec Miata where close racing is the norm, that design element can become problematic, turning what would otherwise be inconsequential contact into the end of a race.
Note: Welding is required for installation.