I found out something about the knob. Apparently, the knob adjusts how high the foot is lifted with the knee lift and lift lever on the back. It doesn't adjust the foot pressure. That is an internal adjustment with a wrench and screwdriver. The knob seems like a "nice to have" feature not a "must have". Also, as far I been able to determine, the newer Juki 1541 machines still do not have that feature.