The Identity Recognition Module was a small item made of crystal and wire. It allowed the Doctor's TARDIS to recognise her pilot, thus permitting her to lock out hostiles from using the console or the TARDIS data banks.
After escaping the Cheetah Planet, the Tremas Master was teleported into the TARDIS's control room and severed the Recognition Module from the TARDIS's systems. Although this did not break the deeper psychic bond that existed between the Doctor and his ship thanks to the Rassilon Imprimatur, the Master was now able to set coordinates of his choosing, while the Doctor did not dare to chase him into the rest of the TARDIS beyond the control room, as he could no longer rely on the shifting, labyrinthine maze to obey him. (PROSE: How did this creep get in here, Professor?)