Well locking the rear diff means that both rear wheels must spin equally. This means one wheel will lose traction, causing the other wheel to also. This makes it easier to slide out the rear end.

Counter steer modifications include increasing steering angle and changing gears.
Gears: CS Mods change the drive gears so that more power goes to the rear wheels. You will often see this referenced as a number or percentage. 50%, or 150% countersteer means that there is 50% more power going to the rear than the front. Similar to full-size cars, this allows more realistic "counter steer" drifting in which the steering is able to turn in the opposite direction for a consistent period. This modification is more popular on belt drive cars due to it being easier to swap gears.

Steering angle: To assist counter steering, sometimes the front arms are modified to allow more steering angle. This means it will be able to exit a drift without spinning out while continuing at high speed.

Most other vehicle modifications are purely cosmetic.