Isomorphic controls could only be operated by one user. Such controls ostensibly worked only after identifying the allowed user through genetics or other uniquely identifying properties, such as their biological morphic field, of which the name "isomorphic" was derived from.
One such device was the Master's laser screwdriver. The Tenth Doctor attempted to use it to overpower the Master, only to be ridiculed when he could not. (TV: Last of the Time Lords) At one point, the Doctor's TARDIS key was also said to be isomorphic, (TV: Spearhead from Space) but other, non-isomorphic security features were posited. (TV: The Daleks) Also, the fact that the Sixth Doctor could gain access to other TARDISes using his own TARDIS key seemed to argue against the keys being strictly isomorphic, (TV: The Mark of the Rani) as did the fact that relative strangers like Ben and Polly could effect entry into the Doctor's TARDIS by means of keys not especially made for them. (TV: The War Machines)
Like its keys, the TARDIS' controls were said to be flatly isomorphic, (TV: Pyramids of Mars) or at least uniquely operable by the Doctor. (TV: The Daleks) This fact prevented a shape thief from stealing the TARDIS, even though he had perfectly mimicked the first incarnation of the Doctor. (AUDIO: Mother Russia) However, Sekhmet described isomorphic controls as "a transparent lie, fit only to deceive a child.", i.e. a bluff. The Fifth Doctor seemed to agree with this sentiment by simply stating, "Oh, it worked before." (AUDIO: The Bride of Peladon) Indeed, there were several instances of companions successfully using — or even being actively taught to use — the TARDIS controls. (TV: The Pirate Planet, Castrovalva, Four to Doomsday, Journey's End, The Time of Angels, The Pandorica Opens, Journey to the Centre of the TARDIS)
Kazran Sardick's machine had isomorphic controls. When Kazran told the Eleventh Doctor this, the Doctor initially said, "Fibber". After Kazran operated the controls when the Doctor was unable to, the Doctor conceded the controls were isomorphic. After the Doctor changed Sardick's personality, they no longer recognised him. The Doctor stated that his father, who had set up the isometric controls for him in the original timeline, would not have done so with the altered Kazran. (TV: A Christmas Carol)