What is the purpose of the pre-flush other then perhaps clearing debris?
My two cents: typically the first water washed / flushed from a catchment system / roof is loaded with settled dust, insects, bird poop, miscellaneaous organic material, and pollution particles that you do not want to drink. Rather than overburden and possibly clog a filtration system with the initial dirty water load, you let it drain or flush away for a while before the presumably cleaner water is captured.