Polishing a crank removes small imperfections, tiny surface scratches and cracks and reduces the amount of oil that sticks to it. It also looks pretty on the bench.
A knife edged crank reduces weight slightly and allows the conterweights to slice though any oil that gets in the way.
Nightrided was a programme with KITT and Hasslehoff. Ohh, you mean nitrided
Nitriding is done in an oven. The crank is suspended in a closed chamber which which is lowered into the furnace for heating. At a determined temperature ammonia and nitrogen gas are introduced into the chamber. The heated gas reacts with the carbon on the surface of the crank to a depth of approx 0.010", making the surface hard.
Nitriding treats the crank evenly from top to bottom and side to side. It sets up a surface tension that stiffens the crank and increases the fatigue life by 18-20%..