It's applied, not blasted
Biocide is applied by brush or low-pressure spray, never loaded into a pressure washer's soap reservoir for application under pressure. The point is to coat the surface evenly and let it work chemically, not to force it in with force.
Dwell time is the part people skip
A biocide treatment typically needs to sit on the surface for around 7 days before the area is brushed, scraped or gently rinsed clean. Rinsing it off early undoes most of the point of applying it in the first place, since the biocide needs that time to actually kill the moss and algae at root level.
Watch for the colour change
As the treatment works, moss visibly changes from green to a black or yellowish-brown colour, which is a sign it's dying rather than still alive under a wet coating. Once it's gone brittle, a stiff-bristled brush lifts it away easily.
Temperature matters
Biocide generally shouldn't be applied when air or surface temperatures are below around 6°C, which is worth knowing if you're thinking about timing a treatment for deep winter.
Why we usually clean first, then treat
The standard professional approach is to mechanically clean the surface first, then apply biocide afterwards as a treatment against what's left in the pores, joints and textured surfaces, rather than relying on the biocide to do all the work on a heavily soiled surface.
Sources: SoftWash UK, "Remove Algae From a Driveway: Pro Softwashing Tips"; Kingfisher UK, Bio-Wash Moss Remover product guidance.