Commercial prosecco is made in the charmat method, fermentated in large, pressure sealed, generally stainless steel tanks, and the finished wine is transferred to the bottle under pressure. Made with selected yeast and toss in some sugar, and you get the product most people think of as Prosecco. Slightly sweet, clear and filtered, this is a bubbly commercial beverage. Col Fondo Prosecco, however, is initially fermented in a tank, but then transfered to the bottle about 1 degree of alcohol before it is finished fermenting. The remainder of the fermentation takes place inside the bottle, and as the yeast finishes its job, it falls to the bottom, leaving a small amount of sediment behind. Col Fondo bottles are stored standing up to leave this sediment at the bottom, and then decanted into a pitcher at serving time, reserving the last little bit of cloudy wine in the bottle, to be tasted and enjoyed separately
Over at the Jenny & Francois blog, Nick Gorevic writes about the Col Fondo method of making Prosecco in Valdobbiadene.