It's not a tangible thing you can see.
Whatever Enma's case may be, it seems to directly be helping to strengthen Zoro's haki in some way. That effect for Zoro is permanent, so it's a powerup like internal destruction Haki for Luffy is a power up. These things aren't like the Raid Suit, where it only boosts Sanji while he's wearing it, which would constitute that as a temporary powerup. Any kind of new, stronger ability, or strength boost to an already existing ability, is a powerup.
Whatever Enma's case may be, it seems to directly be helping to strengthen Zoro's haki in some way. That effect for Zoro is permanent, so it's a powerup like internal destruction Haki for Luffy is a power up. These things aren't like the Raid Suit, where it only boosts Sanji while he's wearing it, which would constitute that as a temporary powerup. Any kind of new, stronger ability, or strength boost to an already existing ability, is a powerup.
We've seen the hardest Luffy and Sanji could hit before this, and what they couldn't help but fall to in combat. The closest Zoro has come to defeat is running out of breath underwater, or sleep gas, or passing out after whipping someone's ass while nerfed. Is he gonna perform better than he did in those cases? Yes, but that's implied by the time that's passed. That has everything to do with being given time and getting in the appropriate mindset.