I don't know, because it's the least-explained type of haki. It took 200+ chapters from the introduction of Hardening to get an explanation in Dressrosa. I'm not entirely sure there is even a specific visual unless two people are clashing equally and splitting the sky or an ACoC attack lands on someone who isn't defending like with Luffy vs Kaido or Big Mom punching Page 1. Topman's defense is so strong that landing a hit on him might just get totally shut down with no lightning or visible impact at all.
When luffy punches through someone's head like he did vs Kaido and Kizaru this may also be a combination of awakening rubberizing his environment/opps and the "not even touching" effect, because his fist is inside of their head, we can't see if theres a gap or not.
The lightning doesn't seem like the real visual cue because attacks from random people who don't even have haki at all occasionally get black lightning.
