King of Hell (KoH) is a style where Zoro uses overwhelming amounts of CoA haki and combines it with ACoC haki or CoC haki coating.
The bottom right scene is an example of KoH.
The key to using KoH is having the power to flow and add overwhelming, enormous or extremely large amounts of CoA haki to a weapon. Smoke coming from a weapon is the result of overwhelming amounts of CoA haki added to a weapon. Whenever we see smoke coming from a weapon and is not because of a DF or other power, instead because of CoA haki, it's an indication overwhelming amounts of CoA haki is being used.
Zoro first use of enma showed us the results of adding overwhelming amounts of CoA haki to a weapon (sword), smoke.
Enma forced Zoro to release an enormous amount of CoA haki. It was so much haki his arm shriveled up. The result of all of that CoA haki being added to the sword was smoke coming from enma.