What Enma does is pretty simple. It doesn’t allow you to use more Haki than what you actually have. That’s impossible, and may actually kill you.
Haki is a finite source, right? We know that, Luffy‘s said at much in Dressrosa when he ran out against Doflamingo and had to wait 15 mins for it to recover.
So you have Zoro. Zoro hasn’t attacked anyone yet. At this stage, he has his Haki at 100%. He’s fresh and ready to fight.
Say he does an attack like Onigiri, using Hardening, but not using Enma. His Haki reserves then go down 5%, he’s used 5% of his Haki.
Zoro then waits five minutes, and his Haki reserves are back up to 100%. He decides to try and cut a tree, using Enma, but without using Haki.
Enma does not then make Zoro use 200% of his Haki. That‘s impossible, and i think would probably kill him. What it does is draws out all 100% of his Haki, despite him wanting to use 0%. That makes his attack much more powerful than intended, destroying the entire cliff that the tree is on.
It’s also a terrible way to fight, as Zoro is now down to 0% Haki reserves and is totally useless. He has to fight with the sword to try and recover some Haki. He has cut more than what he intends, and a good swordsman only cuts what he wants to cut. That’s the heart of Ryou, what Zoro did against Mr 1, and what Oda brought back in Udon as the first step for Luffy learning internal destruction Haki.
So when Zoro’s using Enma, every attack is a fight with the sword. He’s having told it back from taking everything he has and leaving him useless. That’s why his attack failed to the damage he wanted it to this chapter.
But the advantage of Enma is that by training with it, and using it, you can gradually increase your ability and strength in using CoA. It’s a bit like weightlifting, if a beginner tries and go for the heaviest weight they’ll snap their arms in half. Work gradually up though, and their strength will grow incrementally with it.