Technically, you don't backwash the pool, you backwash the filter. This is assuming you have a sand or DE filter. The filter valve should have a backwash setting. Turn off the pump, set the filter to backwash, and turn on the pump unitl the water runs reasonably clear.
If the pool is really dirty then it is best to set the filter to waste.
Drain/Backwash the pool, clean the inside, and add fresh water.
You leave your pool filter set to filter hook up hose with a vacume attachment vacume with filter set to filter or to and then backwash you filter if you have a sand type filter you can also vacume while filter is set to waste but you will need to add water when done. If you have a cartridge type vacume set at filter then remove filter cartridge and clean or replace.
If you're talking about your pool filter, it should be set to filter or run. Use the bypass or recirculate setting if you wish to circulate the water without having the water pass through the filter. Use the backwash setting to clean the filter.
You have two options when vacuuming a pool. If it a normal weekly vacuume you will set your filter valve to the filter position. This will send the water through the filter and back to the pool. Once your done vacuuming, you then backwash and rinse the filter. A good ratio is 3 min to back wash and 30 seconds to rinse. If your pool is extremely dirty like at the begining of the year, You can sometimes set the filter to the waste position. This will bypass the filter and send the water down the backwash line. This lowers the water in the pool. If your pool is extremely dirty and you try to vacuum with it set to filter, the filter will not catch all the dirt and some will end up back in the pool in the form of dirty or cloudy water. As a pool profesional I always strived for clear water first. You can't clean what you can't see.
Set your filter to Backwash or Waste, but don't leave it. Empty out what you do not want and return filter to Filtration
It sounds like either your valve is not set completely to "filter" or your valve's seals are degrading. If the valve is completely set to "filter", then either replace the valve, or plug the end of the backwash hose.Here's a good place to look for new backwash filters: http://www.swiftpoolsupplies.com/category/product-list/backwash-valves
It sounds as though one or more of the laterals in your sand filter are in need of replacement. when these become worn they do not hold back the sand in the filter and it ends up in the pool.
Probably because it is clean, leave it on the filter setting for a while and you will find that the pressure will increase as the filter gets dirtier, when it get high (usually in the red ) you will have to backwash.
A pool with a sand filter or for that mater any filter is set up to push water through the fitter to be filtered. when the filter is clean the gauge in front of the filter Will have a low pressure reading. As the filter gets loaded with more of the dust and dirt that comes from the pools water it becomes harder for the water to pass through the filter and the pressure on the gauge goes up when the pressure get high on the gauge it is time to backwash the sand filter or to clean the cartridge filter. When the pressure on the gauge is high the suction from the pool is low. When the suction from the pool is low even when the pressure on the gauge is low there may be a problem with the pump, or a blockage between the pump and the pool, To increase the amount of pressure on a clean filter the only thing that can be done is to enlarge the pump.
You can set up a siphon hose if your pool is on a hill, otherwise, buy a pump. Hatawa