The one everyone confuses with pitch shifting — and the difference is the whole lesson. Pitch shifting multiplies every partial by a ratio, so 200·400·600 → 300·600·900: the ratios stay 1:2:3 and it still sounds like a note, just higher. Frequency shifting adds a fixed number of Hz to every partial, so 200·400·600 → 310·510·710: the ratios break and the tone turns clangorous and bell-like — inharmonic. It's done by building the analytic signal (the sound plus a 90°-shifted copy of itself) and multiplying by a rotating phasor. Tiny shifts give slow phasing and that eerie barber-pole glide; big shifts wreck harmonicity entirely.
Add the two for the total shift. Fine ±50 Hz is where the barber-pole and phasing live; the coarse slider is where harmonicity dies.
play the source (a s d f …) or start a drone, then sweep the shift and listen to it go inharmonic