By decanting, I really mean using an aerator. It is basically a funnel that is plugged-up in the bottom, but with little holes for the liquor/wine to drain through and aerate. Here's a link to a pic:
Wine Enthusiast Aerating Funnel with Removable Screen - Wine Enthusiast
A decanter is normally what would receive the liquid as it is poured through an aerator, although any empty bottle would do. You can do this multiple times back-and-forth between bottles. And of course, this opens it up by allowing some of the alcohol to evaporate, thus concentrating the flavor of the spirit. A decanter alone also opens a sprit up by having a larger basin than a normal bottle, and allowing more surface-to-air exposure, and again the evaporation of alcohol. The spirit has to sit longer in a decanter to open-up, whereas with an aerator it happens much more quickly, especially if it is done multiple times.