Originally Posted by Scimmia
From what I've read, there are different stages that the yeast go through. When they have oxygen, they are mostly using the oxygen and sugar for reproduction. After they're out of oxygen, they really start producing alcohol and CO2.
Not to mention your finished beer would be HORRIBLY oxidated.
When cells consume O2 and sugar it is called an aerobic reaction and Glucose is converted to all the way down to CO2 and H2O, producing lots of ATP that the cell uses for energy. This is when the yeast will expand, or rapidly reproduce. ( ~Double every Hour)
When cells lack O2 (as in the fermenter) they run an Anaerobic reaction that is not as efficient as the aerobic reaction, breaking down the glucose to CO2, CH3COH (Ethanol) and H2O, producing only a couple ATP/ Glucose.
because this is vary inefficient yeast will not reproduce in these conditions. The starter is done so you can have the maximum amount of yeast in the fermentation vessel to convert your sugars to Ethanol and CO2, as quickly as possible. If you under pitch then the yeast you pitched will have to do more work, and as they age in unhealthy conditions they can impart off flavors into the beer.
If you do agitate the yeast in the fermenter you will probably have to stop and wait a couple days before racking it to the secondary, but I don't think it is too good of an idea to agitate the yeast cake, its kinda the sewer for the yeast.