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This is pumping water backwards through the filters which can include sand filters. If you backwash your pool on a regular basis then it can be a form of preventive maintenance which will keep your pool in good working order and prevent any unforseen problems. Backwashing removes trapped particles in the system.
To Backwash your pool you need to take off the filter and then the water is drained to a level that is above the surface of the filter bed. Then compressed air is pushed up through the filter material causing the filter bed to expand and forcing the trapped particles out. Then clean backwash water is forced upwards through the filter carrying the particles out.