Dynamic stiffness has been around for may years. That the original literature is from the authors you mention is suspect. Quadrature is a term they liked, perhaps there is an electrical connection. On the DVFIIs, there was output on the back label quadrature - if I remember correctly. One needed to connect the outputs correctly to generate polar plots on an x-y recorder.
One could think of this as a complex stiffness.
Important for dynamic stiffness is that these are frequency dependent (in general). One force acts in the same direction as the displacement, and one force acts orthogonal to the displacement. If you like to look at velocity or acceleration, remember that velocity leads displacement by 90 degrees (at a single frequency) and acceleration leads velocity by 90 degrees.