I would like to learn what happens inside the sand filter when you add a small amount of DE.

Due to the leak in our pool it sat for 4 days without filtration. I added bleach and mixed it in manually but couldn't run the pump until the water level was high enough again. The dirt in the air settled on the bottom and billowed up in dingy clouds when I brushed. Having read about adding a small amount of DE to aid in filtration I did just that: I put less than 1/4 cup in the skimmer.

After about an hour the filter pressure had risen by 3 PSI. I was running the pump on low. After 3 hours or so there was no water coming out of the return though the pump was running and not particularly hot. I switched to the high setting and water flowed out the return again at what felt like its normal pressure. At the gauge, pressure had gone up by a total of 4 PSI now.

The pool was amazingly clean after running the water through the filter for 4 hours!

Am I correct in thinking that the DE layers itself over the laterals and incoming water is pushed through this DE layer into the sand? At low speed, is the pressure not high enough to push the water through the DE layer?

I backwashed yesterday and got rid of some very brown water but it cleared up much faster than it usually does. Pump clean pressure is back to normal (15 PSI on high) and the water is very clean.