Michael backwash after cleaning to.
Doing it before just improves the flow by clearing the filter if performance is marginal.
The debris goes where ever the waste pipe goes, best to follow it. Technically it reverses the flow through the filter, lifting the sand bed and allowing trapped dirt to be flushed out of the filter.
Recirculate just uses the pump to push the water around the circuit and the pool but bypasses the filter bed.
The sand in the filter doesn't wear out but does get stuck together with body oils, sun creme bacterial bio film etc and it forms into clumps (mud balling) this produces channels in the filter that allow the free passage of water through the filter. Once a year, close off the valves and clean the sand with a surfactant cleaner, moving the sand around with a pole to break up any mud balls but be careful of the laterals in the bottom. Insert a hose and flush out all the gunk and cleaning agent, you'll be amazed at what comes out. Re assemble the filter cover and give it a good backwash again to flush any remaining cleaning agent before putting it back into filter.
Domestic filters are very bad at backwashing in general. Partly because a one speed pump is fitted so dirt is stuck down with the same force as you are trying to remove it. The sand bed needs to expand by at least 25% and be back washed for long enough to see the water running clear in the little sight glass on the valve (if fitted)
A soak in chlorine dioxide over night also does wonders for shifting gunk, Chlorine dioxide is available in tablet form so can be sent via the post.
Sand is OK as a filter medium but it is limited to around 20 microns at best, often 30-40 microns depending where the sand comes from. AFM glass is a big step forward in filtration filtering to 4 microns so the water takes on a brilliant look. The more impurities we remove from the water the longer the chlorine will last and AFM's surface unlike sand is self sterilising so bacteria cannot grow on the surface so it doesn't mud ball, or channel. Have a read on the thread I posted