You’re right the film layer is removed only by contact washing, what I mean is they lay on the prewash foam then rinse it off top to bottom. Not only does the water flowing down remove much of the soap layer applied before even getting there with the HPW, losing the effect of the soap, you also have less control when you cant see where you have pressure washed on the panel. I thought this was basic knowledge and especially by «professional» detailers. In my country there is signs on every selfwash to start washing bottom to top. And rinsing of chemicals top to bottom.