Arguably, the most important part of cask conditioning is venting; the controlled release of co2 and yeast from the cask bringing it into perfect condition.
When the cask has rested, it has undergone a secondary fermentation and produced carbon dioxide at very high levels. This needs to be released, in effect lowering the carbonation, and, ideally, leaving the drinker with that elusive mouthfeel that we look for in a properly conditioned pint.
There should be no prickly sensation on the tongue, as found in a beer like Orval. Conversely, the beer should not be flat. A drinker should be able to swirl the glass of ale releasing fine bubbles of carbon dioxide.
A cask just vented
Venting IS the art of Cask Conditioning. Too much, and you're left with flat beer. Too little, and the ale doesn't have the mouthfeel sought after in cask ale. One of the nice things about traditional British Real Ale is the low alcohol, yet tremendous flavor. Too much carbonation, and they seem weak. Bland. Boring. Get it just right, served at the proper 52-56F cellar temperature and they are about as close to perfection as a beer can be.