To see why these pieces of equipment are useful in propagating yeast we must venture into a little yeast biochemistry and metabolism for an explanation. Yeast are wonderful survival machines with the ability to survive in both aerobic and anaerobic environments. In an anaerobic environment such as brewers wort yeast use the sugars to produce energy with ethanol produced as a glorious waste product. If however yeast find themselves in an aerobic environment they respire much like us producing carbon dioxide and water. With this is mind we can see that it is very important that the environment that yeast finds itself in during beer fermentation must be anaerobic otherwise we would have the horrifying situation of no ethanol production, vast amounts of yeast cells and a strange liquid that couldn't really be described as beer. Thankfully, there is no way that brewers can accidentally aerate to such as extent that ethanol production is completely inhibited. However at the beginning of fermentation a certain degree of oxygenation is essential to set the yeast up for the work they have ahead. Before pitching, yeast is usually stored for a period during which it has to rely upon its own reserves to survive. These reserves are rapidly depleted during storage and yeast is in no position to effectively ferment beer when it is pitched. A ready supply of oxygen at the beginning of fermentation is necessary for the yeast to replenish itself and and produce sterols to make the cell wall permeable to the wort constituents. Also during this period there is an increase in the number of cells, as cell proliferation occurs at the expense of ethanol production in the aerobic environment. It is after this stage that things kick off and a vigorous fermentation ensues with lots of gas production accompanied by copious foam and ethanol, of course.
With this little bit of information in mind the objective of yeast propagation can be laid out. Liquid yeast for home brewing is in a depleted state, ready and waiting for a chance to grow. The number of cells in the average yeast pouch is insufficient to pitch, so a propagation step is needed. This step is essentially the same as the first aerobic stage of fermentation and this is where my equipment comes into use. Sterile aerated wort is placed in a flask and the depleted yeast pouch is added to the wort where it is continually mixed on the magnetic stirrer. The aim of this exercise is to produce as many yeast cells as possible to meet the pitching rate required for the gravity of the beer, which is easily done in an aerobic environment where the yeast will proliferate rather than produce ethanol. The mixing plate keeps the cells in contact with the wort ensuring maximum use of the sugar available, and also mixes the oxygen making it available to the yeast. Studies suggest that greater cell counts can be achieved with this method and it is also a hell of a lot handier than having to shake the flask every time you walk by it.Generally a much faster start to fermentations can be had this way because the yeast is ready to go when pitched which has many advantages in the home brew environment, the main one of which is not having a big bucket of nutritious sweet wort laying about the place just screaming out for some opportunistic bug to come along and ruin. Once the fermentation kicks off the pH starts to drop rapidly creating an acidic environment that suits only the hardiest of bugs (and they sadly are quite common) and the increase production of ethanol contributes further to the inhospitable environment.
I'll leave it at that because the other great aspect to using liquid yeast is the complexity they bring to beers, which is often greater than that of dried strains. But this is a touchy subject with home brewers because some believe that dried strains are just as good. I don't agree, and that debate is worthy of its own post at a later time.