Smooth is a typically Airwindows invention: a plugin that strictly alters texture, without affecting frequency or amplitude. At least, that’s the intention, and the result’s pretty convincing.
Used on a distorted guitar, it really brings a softer texture up to the point where it breaks up.
Used on something like an over-zingy acoustic guitar, it’s really quite amazing. You can dial in the ‘well-behaved’ and make the guitar sit back and be soothing—even if it’s something alarming like a direct piezo input. It goes totally natural, obediently, without losing desirable sparkle.
Used on a drum kit it’s even more startling. You get a beefy upfront kick, but snares drop right back and become vibey and ambient. Pushed too hard, drums squish. Set just right, Smooth auto-balances drums, presenting a mellow and sophisticated picture. Not for every mix, but a very slick option. The sweet spot is very narrow, but amazing.
Used on a bass, if it’s a noisy buzzing rowdy bass you can push it right to the point of distortion and it does neat things with the attack, defining it nicely. In general with a bass you can tune it so that you’re right between the rawer, plain sound and the fuzzy, overloaded sound, to get an amazing articulate tone that really stands out.