Below is a link to a nice study of fermentation, it's a bit of a scientific approach and I know some folks on here don't like the science so be warmed! It's defo worth a read as it makes clear what the yeast do, when and why.
http://wineserver.ucdavis.edu/indus...on_management/wine/problem_fermentations.html
http://wineserver.ucdavis.edu/indus...on_management/wine/problem_fermentations.html