It is a process of cleaning exterior surfaces without the use of a pressure washer. Pressure washers are not bad to use, but in my experience for certain jobs, low pressure is better. I use low pressure in most cases where there is staining from organic and light inorganic matter in places such as: vinyl, porches, stucco, asphalt shingles, metal roofs, and painted surfaces.
Low pressure is gentler on these surfaces and is less likely to cause problems often associated with pressure washing like: water intrusion at doors and windows or damage to these surfaces.
Other benefits include that it lasts longer than pressure washing since the chemicals kill the algae, its spores, and root system. This is because pressure washing doesn’t kill the algae, leaving behind what the plant needs to regrow. Our process for low pressure washing is to prepare the home for the wash by protecting the landscaping and plant life by rinsing with cool water, applying the solution from the bottom up to prevent any streaking, then rinsing from the top down after a dwell time of an hour. The chemicals that are used are varying strengths of detergents, surfactants, and sodium hypochlorite or bleach. They are all safe for use on the substrates that they are used on in the correct strengths. They are used as diluted, as is still effective, which is commonly 1-3%. These are all common practices of the trade and they leave wonderful results with pressures similar to that of your water hose. They are also recommended cleaning measures by many manufacturers and other sources such as these links for roof shingles, vinyl siding, and metal roofs.