Backwashing

Here's a snippet from a recent email I got from "fireyourpoolguy.com" --

"What is backwashing?Sand filters trap dust and dirt, as the name implies, in a bed of sand. When the filter has accumulated a large amount of dirt, the water cannot pass freely through the sand and the filter loses efficiency as the pressure increases. Backwashing sends water backwards through the filter and flushes the trapped dirt out. After backwashing, you will notice an increase in return pressure to the pool as well as a drop in pressure inside the filter (simply take peek at the pressure gauge on the filter).
---------------------------------------------------Note***If you have a cartridge filter, backwashing is not done. The elements need to be cleaned manually by taking the filter apart and spraying them down every 7-10 days."