How does backwash work?

Answer 1

On your typical sand filter the water basically goes in through your multi-port valve and works down through your silicat sand (Filter median) and into your laterals to your pool or spa. During backwash, the water is taken from your spa and ran through your laterals first and agitating the silicate (filter median) and pulls most of the "waste" that was trapped by your filter and flushes it out through your drain hose off your multi-port valve.

This does two things, one of which was mentioned prior and the other is that it agitates the sand in the tank and removes any channels that have been bored from water being pulled down from the top. It cleans the filter median as well as gives a reguvinated filter without pitting or "worm holes".

You can also use a cleaner that you put into the skimmer basket and shut off the main drain lines and backwash as normal. This helps remove some of the soaps and fats that are attached to the filter median and don't get agitated loose through normal backwash.