What is DLC coated engine?
Some people polishes the crankshaft to make the engine work faster.
Question is , how do we polishes the crankshaft? And what will it gain?

DLC is a diamond coating. It is very low friction and helps when the rotating speed goes beyond the limit of the oil.

Crankshaft polishing is more to not disturb the flow of the fuel, there are different thoughts about it. Polishing makes the flow faster, making it rough does make a better mix of fuel and air.