The differential is mounted in the car with some surprisingly flexible rubber bushings. Replacing these with stiffer Prothane polyurethane will cut down on differential movement, which actually makes your shifting more precise due to the way the Miata's drivetrain is mounted. There's an increase in vibration, but most people don't mind it at all. A definite plus for competition cars - you don't want to hit 5th instead of 3rd in the heat of competition!
Bushings fit 1990-2005 only.