Back to the original question.
You will get an original pitch up indication of 15 degrees NU.
This should result in a V2 plus 20 speed.
If you underrotate, you are not following the FD, but the FCC does not know that. In fact this computer is made to believe that you as a competent pilot are following the FD command.
The computer only learns quick that a 15 degree NU command results in a higher than V2+20 speed.
Therefore the clever computer will give you now a 20 degree NU command, to try to satisfy the speed requirement.
When you now start pulling on the stick, speed will come back. The now happy computer will command the FD to come down, and meet you somewhere halfway..
If you rotate as you should, you will not see The FD showing you 20 degrees NU, unless you ferry the plane with TO trust.