Back flushing is a process done to espresso machines to clean them: a filter basket with no holes (a “blank” basket) is inserted into the portafilter so that when the machine is activated, pressurized water cannot escape and is instead forced back into the machine to “flush” it. Often, back flushing is done with some type of coffee cleaning detergent in the basket. A typical back flushing protocol is to put coffee cleaner in the blank basket and back flush 5 times, then rinse the cleaner out and back flush 5 times without no cleaner. Note that not all machines can be back flushed. View Cleaning espresso machine video.