When you disconnect the PFC, you are removing all envelope protections. (Direct Mode). Your control inputs are translated to a voltage to the ACE's (Actuator Control Electronic) which will move the relevant flight control as much as you command them , irregardless of airspeed, bank, etc.
I suspect the reason for the disconnecting for unreliable airspeed would be to do with stall warning/stick push or other floor protections which may limit/override your input.
Have a look in the SDS 27-02-00 page 15
Last edited by LME (GOD); 22nd Jan 2013 at 13:27.