The gluten content of the dough will trap gas when stretched by kneading. The retention of CO2 in the dough causes it to expand to give the dough more volume. By knocking down the dough and forcing the gas out, the dough can be kneaded to stretch the gluten and redistribute the yeast (which will have multiplied) to give a better and more evenly distributed gas retention and a finer finished loaf.
Yeast will work best at about 30ºC and will be killed at over 40ºC. It is therefore important that the yeast is not exposed to very hot water. Baking will kill the yeast and denature all of its chemical enzymes.
Dry yeast - has a very long shelf life and will become active when mixed with water. Keep relatively cool and dry to store this type of yeast. Add to the dry ingredients before adding water.
Brewers yeast - is a cake of live yeast which needs breaking up and soaking in warm water with a little sugar for about 30 minutes before use. This type of yeast has a relatively short shelf life and is not in common domestic use since the production of dried yeast.