Enma is said to arbitrarily draw out a user's Ryuou (Haki) and cuts more than required. From this statement it makes sense to assume that the source of Enma's DC is that overload on Haki but what I don't get is why Zoro was able to get said Haki back from Enma. It doesn't make sense for him to have gotten the Haki back if it was used for the attack on the cliff as it should have already been exhausted. The only way it makes sense is if Enma takes the Haki as a payment for the destruction it causes (in which case Zoro getting it back makes sense as said Haki is still in Enma's possession) but that doesn't seem to be the case going of what Kinemon's explanation.
I think it's because of sword will.
Ryuo is basically channeling your haki from within and surrounding.
Enma has a sword will. It sucks in ryuo from swordsman wielding it in unrestrained manner. This can be infer from Zoro hand getting thin and all that clouds.
However, in next panel we see Zoro arm again getting buffed up which means he regained ryuo what he lost.
So, initially it was sword will which took ryuo from Zoro as Zoro has no control over enma nor on ryuo but once Zoro controlled and tamed enma it became Zoro's will. That is enma now can't suck in Zoro's ryuo at its own will.
Also, Zoro took back the excess ryuo that Enma sucked in first place. That's why his arm got buffed up.
This is what I understood. Chapter would make things more clear.