The differential is mounted in the car with some surprisingly flexible rubber bushings. Replacing these with stiffer bushings will cut down on differential movement, but they're a headache to install. Looking for an easy answer? Enter these bushing inserts. These inserts, made by SuperPro, fill in the voids in the stock bushings to minimize the movement of the differential - this actually makes your shifting more precise due to the way the Miata's drivetrain is mounted. They're a pretty easy install, as you just have to lower the differential enough to slip the smaller part into place, then put the bottom piece in place. With full replacement bushings, you need to remove the differential altogether then remove the stock bushings (which typically requires fire. Seriously.). A definite plus for competition cars - you don't want to hit 5th instead of 3rd in the heat of competition!