In an environment without air, like the bottom of a wine barrel, yeast can release sulfide compounds that don’t smell so nice. This is a form of “reduction.” “Racking” is the process of moving the wine from its barrel to another barrel or a tank. The process removes sediments that drop at the bottom of the barrel that can give bad flavors to the wine. Once empty, the barrel is cleaned with hot water and steam to remove sediments stuck inside the barrel. The barrel is left to dry and wine is put back in the barrel. An added benefit of racking is that it adds some oxygen (oxygenation; a good thing) to the wine, which reverses reduction and helps the wine mature. Done properly, this will allow more precision and clarity in the wine.