An explanation of the Charmat method by Azienda Paltrinieri, producer of sparkling Lambrusco wines.
The Charmat method requires a second fermentation (or refermentation) to create a sparkling wine.
So it starts with a base wine, that already underwent the first fermentation from grape must to wine. The carbondioxide (a byproduct of yeast fermenting sugars into alcohol) created during the first fermentation could escape freely because the tank wasn't pressurized.
For the second fermentation the base wine is put in an autoclave tank, a pressurized temperature controlled tank. Then they add a calculated amount of sugar (in the form of preserved grape must from the same vintage), resulting in a calculated amount of carbon dioxide during refermentation, that unlike during the first fermentation, cannot escape the tank and is now dissolved in the wine. So now you have a sparkling wine.