When your pool has a build-up of very small particles or debris that you don't want going through your filter (like algae), you can send the debris through the vacuum and directly out of the pool by vacuuming on the "waste" setting. Vacuuming to waste can only be done with filter systems controlled by a multi-port valve, commonly seen on sand filters. Normally, vacuuming is done on the "filter" setting, as "waste" sends both water and contaminants out of the pool through the backwash hose.
Because vacuuming your pool to waste will lower the water level, you'll want to be filling the pool simultaneously. Place your garden hose in the pool, positioning it so the stream of new water won't stir up the dirt you're planning to vacuum. If the water level gets too low (below the skimmer), you'll have to stop, refill and begin again later. If you have a heater or pool cleaner, turn it off.
Connect your pole and the swivel end of your vacuum hose to the vacuum head and slowly lower the vac head into the water. Fill the vacuum hose completely with water and connect the other end to the skimmer vacuum plate. Seat the vacuum plate securely on top of your skimmer basket. Clean out your hair and lint pot (pump basket); a full vac hose and clean hair and lint pot will ensure plenty of suction. Normally, if hooked up correctly, the pool's filter system provides plenty of suction because of the constant circulation of water. When vacuuming on waste, the water isn't circulating, so you'll need all the suction you can get.
Once the vacuum parts are ready to go, turn the pool pump off and dial the filter to the "waste" position. Once you turn the pump back on, the pool will begin emptying, so work quickly. Be careful not to break suction by pulling the vacuum plate loose. A buildup of debris in the skimmer basket or hair and lint pot will cause a marked decrease in suction as well; if this happens, you'll have to shut the pump off again to clean them out. If you have additional skimmers, close them, and if you have a bottom drain, close it.
Begin vacuuming in the deep end, if your pool has one. Don't forget to watch the water level. Allowing the water to get too low can make your pump run dry, and that can cause serious damage. If you finish vacuuming and need to add more water, turn off your pump, disconnect your vacuum equipment and clean out your baskets. Once your water level is back up, turn the filter to the "filter" setting and switch on your pump.