Sahil Sura
Ansys Employee

Hello Aitor,
When you link the systems, the results you obtained are with considerations of deformations, stresses, and other parameters from the results of stress analysis as an input to the modal analysis. 
If you want to verify the same, you can model a simple cantilever beam and get its mode shapes and frequencies. Now you can duplicate the system, add forces, to induce stress in the same, and then link the modal to the system, you may want to compare the results.
What I would suggest is to choose a simple geometry where you can calculate the modes by hand calculations too!
But be rest assured that when you link the analyses (share a solution of the first system to the model of the second) the results of the first analysis serve as inputs to the second.
You also might want to check the following resource explaining the pre-stress modal analysis - How To Perform Prestressed Modal Analysis — Lesson 2 - ANSYS Innovation Courses 

Hope this helps.

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