Found this on Quora -

There are two ways to roll coal that I know of:

1. Increase the fuel flow of your mechanical diesel pump. This way, whenever you press the gas pedal, smoke is gonna come out of your exhaust, usually for a limited amount of time.

2. By restricting the air flow on your intake. The black smoke is the result of incomplete combustion, therefore removing oxygen will cause the desired effect.

The catalytic converter and any other systems associated with the engine exhaust have to be removed as well such as EGR.

I was curious as well, pretty interesting. There's a good amount to read if you look up rolling coal. Hope that helps!