Originally Posted by jcaudill
Uhm - at what part of the process are we talking about here?
Good question; moving the yeast around is not part of the regular process. But it is handy to know about in certain situations.
Lager yeast is a bottom feeder and will drop out of the beer when it is cold, getting lazy or dying from a high alcohol environment. Yeast will work in solution and from the bottom, but as more layers are added, the bottom yeast no longer have contact with the beer and stop working. Thus, rousting up the beer brings the lower layers back into contact with the beer. The exception would be the yeast that are dead, rousting them wouldn't help.
When fermenting lagers or Boch type beers, which is also a lager, your temperature will often be around 50F. The yeast can handle this type of temperature, but require a larger amount of yeast to accommodate those yeast that slow down and sometimes drop out of solution.
Your most ideal situation is to have a large enough yeast slurry that is healthy enough to handle cold and higher alcohol environments. There are yeast calculators on the web that will help you determine the most beneficial amount of yeast to use. In addition, a good amount of oxygen at the beginning of the fermentation process will strengthen the yeasts outer layer making the yeast less permeable to alcohol and therefore lasting much longer in a high alcohol environment.
In conclusion, rousting the yeast is a good way to move yeast at the bottom back up into the beer solution to hopefully get it back in production.