The backwashing of precoated filters has already been explained because it is more complex than a typical backwash.
The intent of a backwash is to flow liquid in the opposite direction of normal flow, creating a pressure that pushes the debris off the strainer or filter. The debris is flushed to a waste tank or drain.
Normally, to establish a backwash lineup, the flowpath upstream of the inlet to the strainer or filter is closed, the flow path downstream of the outlet is closed, and a drain flowpath is opened.
The flush source is then opened and the flow goes into the outlet of the strainer or filter, through the strainer or filter, and exits the inlet to the backwash drain or waste tank, carrying the debris with it.