Most commercial cleaning companies use truck mount carpet cleaning machines.  The machines are usually mounted in an industrial type van and they carry two hoses.  One is a suction hose that is able to bend without kinking and then there is the water hose which allows the chemicals and water to flow through the cleaner, which is usually around a 4 foot long metal wand.

The metal wand usually has 4-5 nozzles in the bottom of it which allows the water and chemicals to be evenly dispersed in a fan-like pattern.  You can usually tell when a nozzle is clogged because of the interruption in the pattern.  They are fairly easy for the technician to unclog.  He simply twists the nozzle off, uses a pin and cleans out the jet.  Mostly hair and very thin fibers cause the most clogs.

The technician usually will vacuum first and then will clean and rinse, trying to suck as much water out of your carpet as they can to help with lowering the drying time.  Most machines are set to a certain psi pressure to prevent the machine from sending too much pressurized water into the carpet and padding.  Our company sets ours at 400psi.