Understanding the cleaning process is essential to the effective use of a pressure washer. A pressure washer cleans by creating a high pressure stream of water that hits the surface with a large amount of kinetic energy, thus removing the dirt mechanically, while providing a constant flow of water to wash the dirt away. Often dirt is mixed with grease or oil, or other substances, which chemically bond the dirt to the surface you are trying to clean. Sometimes even the force of high pressure water needed to break these chemical bonds is insufficient to get the surface clean without requiring excessive pressure, that is either impracticable or would damage the surface. Adding a detergent, or other cleaning chemical, or using hot water, while agitating or brushing the surface, significantly enhances the cleaning power of the pressure washer.
Detergents and soaps are used for cleaning because pure water can't remove oily, organic soiling. Soap cleans by acting as an emulsifier. Basically, soap allows oil and water to mix so that oily grime can be removed during rinsing.
Detergents cannot accomplish much until some mechanical energy or agitation is added into the equation. This mechanical energy is provided by the pressure washer. As the detergent emulsifies the grime, breaking the chemical bond, the blasting effect of the pressurized water strips the dirt from the surface, while the flowing water rinses the detergent and soil away. Warm or hot water, or steam, melts fats and oils so that it is easier for the soap or detergent to dissolve the soil and pull it away into the rinse water.