Tije's Transparent Portafilter clip3 from Frans Goddijn on Vimeo.
Tije de Jong made this Transparant Portafilter. Here are a few first extractions with it. Sometimes at the end when the pressure drops (backflush) the puck expands surprisingly. Also, it seems a firm and level tamp helps a lot to get a delicious extraction, at least in this test setup.