I posted the quetion at Cineversity too, got an answer from Dr Sassi. For the record:
"My understanding is – that the joints that drive the points for PSD must be in the Driver list. With that, the resulting Joint-pose is part of the recorded PSD result.
With two joints producing pose one and then in pose two, that would result in the need for a definition of each pose, as the angle for the Shoulder changes."
I now have a PSD correction on the elbow with the shoulder and the forearm set as the driver and (very important) the PSD interpolation mode set to Global Strength in stead of Per Joint. "This mode will apply a deformation percentage based on the global delta variations of all the joints. All their delta values are averaged as a common strength value" the manual says.