The WDT, or the Weiss Distribution Technique is named for John Weiss, who developed the technique back in 2005 as a way to compensate for grinders, especially smaller home grinders, that produced excessive clumps.
It can help compensate for those shortcomings by evenly distributing coffee in the portafilter to promote more even extractions and better espresso.
As you turn, the needle effectively break up clumps and distribute grounds evenly improve brew flavour