The idea behind it is either to pre-soak and loosen the grime before a bucket wash or even to reduce the need to use a wash mitt at all.
I have not got a pressure washer but use a pre foam soak before washing with the Gilmour and a hose using a mix of small amount of snow foam and Meguiars Hyper Wash. Works best on dry car, i,e dont pre rinse with water first, spray the foam on the car and let it dwell and run down the car taking any dirt with it. While it does that I fill some buckets . Then I just wash normally using the two bucket method and wash mitts. I don't trust it to clean the car stand alone on Black.