What does it mean that haki cancels out a DF? Blackbeard is the hardest to counter with haki

[Warmmedown]

If Law said strong enough haki cancels a fruit's power, does this mean if Sanji activates stronger haki than what Enel activates, he won't be hurt by the electricity at all? Or just that he'll diminish the electricity's effect by the difference in their haki? Did Law just mean conqueror's haki or armament haki too?

Does it mean that if Shanks emanates haki a big distance from himself towards Sabo's flames, Sabo's flames will disappear? Or sending it towards Blackbeard, his void will literally disappear?
Blackbeard's fruit seems the hardest to negate with haki, since it's gravity and that's a constant pull. Fujitora's could be countered with haki at the moment when he starts to alter its gravity (eg imagine it's Fujitora trying to pull Aokiji's ice leg away), whereas Blackbeard's you'd have to negate for the entire fight if he's set up a giant black hole like versus Ace. Aokiji would have to coat his ice prosthetic (which isn't really part of his body) in haki the entire time to just not get it pulled off. It would be tiring and leave less haki for other things.

 

[SSJ2]

In this context it sounds more like you can overcome the effect a devil fruit puts on you. Like it might be able to counter the effects of Perona's fruit. I don't think it makes you immune from any devil fruit attack though, especially physical based attacks.