It goes without saying, in order to properly clean carpets and fabric upholstery a detailer must know the kinds of soil that they are dealing with. Otherwise, how can they know how to attack it and rid the carpets and upholstery of the unsightly contaminants? They cannot.
That said; let us identify the kinds of soil that are typically in carpets and on fabric upholstery:
a.) Dry Soil
b.) Oily Soil
Dry Soil: It is estimated that 85 percent of the soil in carpets especially and fabric upholstery is dry soil.
Oily Soil: Fifteen percent is oily soil attached to the carpet fibers.
It should be clear that a very thorough and deep vacuuming is done to obtain clean carpets/upholstery. However, many detailers still simply slop the shampoo/water mix on the carpet without vacuuming at all, or at best, do only a cursory job of it; incorrectly assuming the shampoo will get the carpets clean. All this does is turn the dry soil into "mud" which is even harder to remove with just a shop vacuum.
It is common sense that dry soil can be easy to remove. A detailer should always thoroughly dry vacuum the carpet/upholstery. Some use an air blower to help raise the dry soil's dirt and grit out of the fibers.
For oily soil, pre-spray the carpets/upholstery with a foaming-type carpet/upholstery shampoo and let dwell to emulsify the oily soil on the fibers. Note that there is not a great deal of oily soil on the fibers, so it is not necessary to saturate the carpet/upholstery with a great deal of moisture.
Next, friction scrub with a hand brush or, better yet, a rotary shampooer tool which will cause the shampoo to foam up lifting the oily soil off the fibers. Now is the time for the heated soil extractor to rinse clean the fibers of the oily soil and shampoo residue.