This RA90 is an attachment for the Stihl pressure washer range models from RE90 through to the RE130 Plus.
Under the splash shield is powered by the water pressure from the pressure washer the arm rotates at high speed with a wash jet spraying water at the surface. This cleans a 255mm / 10 inch path as the brush is moved over the surface.
It's a much more controlled cleaning process and is less likely to disturb any joints between pathing stones and the splash shield protects the operator and surrounding objects. The operating pressure on the RA90 can be adjusted by the control knob on the unit.
It's a faster and easier way to clean patios, paths, drives and large areas of flat surfaces.