What's the meaning of frequency shift in Frequency step?

Hi everyone,

I'm recently studying the topic of structural acoustics. When I practicing the workshop, I found it always turn on "frequency shift" in frequency extraction step. Actually I try to turn off this option but found no influence on simulation results which means same eigenvalue with same mode shape. Hence, I'm curious about the function of "frequency shift" and when should I use this option. Can anybody give me some advice, thanks a lot!