Ok so we now know what Unohana's bankai does... kinda.
It's not explicitly explained. But we do actually get to see her use it against Kenpachi.
spoilers:end of spoilers
It covers her blade and the area they're fighting in with blood. The blood is used to instantly heal her wounds and for long range attacks, seems like she can unleash several slashes and the same time with it. Depending on if one interprets the bit where they've both been reduced to bones as literal then the healing capabilities of this bankai are quite nuts.