In the diagram below, there is a sand filter with a multiport valve in the rinse position.  The water enters through
the top port of the filter and goes through the diffuser.  The water filters down through the sand and the dirt and particles are trapped between the
particles of sand.  The water then goes into the laterals at the bottom of the filter and out through the lower port.  The backwash valve then directs
the water out to the sewer line instead of taking it back to the pool return lines.

This is useful to rinse the filter for a few seconds after the backwash process is complete.  Often, during the backwash
process, a small amount of dirt or debris will collect on the clean side of the filter.  If the rinse setting is not used, then this debris will be sent
back to the pool when the filter is put back into the filter position.