The Portafilter jiggle is a habit I got into very early in my espresso career, a simple practice that goes a long way toward keeping the shower screen and seal clear of used coffee grounds and general gunk.
After, or during a session if you desire, sit the portafilter lightly in place, turn on the brew switch, lever or whatever, then when water begins to flow jiggle the portafilter from light lock to unlock rapidly for a few seconds, works even better with a blind filter.
