Believe it or not, it's letting the cacao beans ROT after they are picked. If you were to eat a cacao bean right off the tree, you would most likely spit it out because of the bitter taste. The key to producing the wonderful taste of chocolate is in the processing of the beans once they have been picked. In the fermentation process, yeast and bacteria secrete alcohol and other chemicals to break down the protein, starch and fat molecules in the beans.
Smaller molecules have a better taste.
The spectacular flavour can then be unleashed when the beans are dried and roasted in the sun.