rate as a real time effect ( Reaper anyone?)

I am experimenting with rate shift in a way which is supposed to be a real time processing of the incoming audio.
Background: if I set feedback to zero, set pitch-shift ( any value), hit overdub and play, I'm getting the incoming signal processed with such a transposition.
Why if I apply the same procedure to rate ( change pitch-shifting to rate-shifting, I'm not getting the relative result ?

thanks
Luca