"Stiff"= resists change from influence
Lower impedance relative to the load.
For PSU's and V regulators or bias V source and audio power amplifers"
% Load regulation error = R
S(source) / R
L (load) * 100% = 1/ damping factor for amp/speaker
If a Si diode's incremental resistance is in the bulk region >> 0.6V or >> 1mA then the low bulk resistance begins to dominate Vf, which depends on size or power rating of diode
"Stewart's Theorem"
Rs ~ 0.5 / P max (rating) +/-50% covers most forward diodes for Si, LEDs and Vce(sat) from ESD tiny diodes to Hockey puck diodes. (Zeners and TVS have an V offset but have a bigger voltage knee factor that varies this rule of thumb.
View attachment 187409
Interactive Simulation
http://tinyurl.com/yqx74wqz
e.g. for sub-Woofers you need high damping factor 200~1000
which means the cone EMF won't affect the voltage applied by this ratio within the amplifier's bandwidth. So the cone is stiff due to the low impedance of the power amp ratio to speaker impedance.
Likewise for transistor bias where 0.1% to 10% may be adequate depending on Specs. for source variation to desired response. (and all tolerances involved).