Hostile Action Displacement System

The HADS, or Hostile Action Displacement System, was one of the defence mechanisms of the Doctor's TARDIS. When the outer shell of the vessel came under attack, the unit dematerialised the TARDIS and rematerialised it a short distance away after the attacker had gone, in a safer locale. (TV: The Krotons) The HADS had to be manually set, and the Doctor often forgot to do so. (PROSE: The Krotons)

While stuck on Earth and attempting to avoid meeting himself, the Fifth Doctor engaged the HADS to move the TARDIS. (AUDIO: The Mutant Phase)

According to one account, the Rani was able to capture the Doctor's TARDIS in a tractor beam because the Sixth Doctor had neglected to set the HADS. (PROSE: Time and the Rani)

The HADS activated itself when two Zygons tried to steal the TARDIS. (PROSE: The Bodysnatchers)

After a particulary violent crash landing, the TARDIS used the HADS to move itself through time as well as space, appearing several days later. (PROSE: Dead of Winter)

While fiddling with various settings, the Eleventh Doctor re-enabled the HADS in time for it to activate and leave the Doctor and Clara in a sinking submarine near the North Pole in 1983. It reappeared at the South Pole a short while later. (TV: Cold War)

Behind the scenes
According to the Make Your Own Adventure with Doctor Who novel, Search for the Doctor, Drax' TARDIS was shown to have a HADS, which was manually triggered when the vessel came under fire. As the Make Your Own Adventure series has multiple outcomes and no mechanism of determining the "right" outcome, this wiki does not consider the novel a valid source.