If an input profile is specified for CMYK, it is attached to DeviceCMYK. All DeviceCMYK colours or process colours are converted to the output colour space using this profile. The overprint settings in the PDF are ignored regardless if an input profile is specified for CMYK or not.
If an input profile is specified for CMYK, it is used as DirectCMYK colour space. Any overprint in the PDF is simulated. If an overprint only involves process colours, the result of the overprint simulation is transformed from DirectCMYK colour space into output colour space. DeviceCMYK colours or process colours that are not involved in overprint are also transformed from DirectCMYK colour space into output colour space.