John
As you are probably aware these machines are designed for "dry" cleaning of carpets. Basically you pre-vac the carpet, sprinkle some impregnated granules onto the carpet and use the machine to agitate them in. During this process some of the dirt transfers to the granules which are then vacuumed up.
There have been many debates over the years as to how effective this method is over hot water extraction cleaning.
If you do a search on the carpet cleaning section you should pull up past posts on the subject.