Thumper Drain back….?

[Capnbob]

In my prior efforts in distillation I did not use a thumper. Would do a simple distill and condense into jars. Then I would run alcohol back thru still again to get more concentration/ proof and purity. Now have a set up with thumper and understand its function and use…..but this one has a lower drain copper tube with a cutoff valve which allows liquid drainage back to still…. Can someone give me an idea of when and how to drain back the thumper keg to the main pot still?.. I’d appreciate advice….