There are 2 ways to "roll coal" on a diesel:
1. You inject extra fuel (diesel) into the cylinder after main combustion - injection even be so late as exhaust stroke, you just need exhaust hot enough for partial combustion. Which gets you the cloud of black smoke. The airplanes at air show are injecting oil into exhaust to get the smoke trails which is probably the extreme example.
2. You inject more diesel into the engine than there is air to burn - this makes more power, more power equals more heat. You can, of course, over do it, and inject so much diesel that things start cooling down again.
What got people in trouble for - the OEMs put EGR and DPF (diesel particulate filters) in the vehicles to pass emissions (Just like they put catalytic converters in vehicles with gas engines). But you can't roll coal with those devices in your truck. So, aftermarket supplied "off road only" products to eliminate the EGR and DPF. Eliminating emissions devices/causing excessive emissions attracted the wrong attention.
I agree with
@lil_Blue_Ford, its dumb to be blowing unburned fuel out the exhaust - you see the price of diesel?! My historic tricks were propane injection which allowed for combustion to start faster and water injection to cool things a bit - the wrath of Dad being far worse for a 16 year old than EPA - rolling coal. being a tip off that you were "experimenting".