The Hostile Action Displacement System, or HADS, 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) However, another function of this was a "dispersal" instead of a "displacement", which would allow for the TARDIS to appear to actually be blown apart, but in fact remaining intact in various pieces. (TV: The Witch's Familiar)
When testing the ship after his exile on Earth had ended, the Third Doctor briefly lost the TARDIS when the HADS was triggered by a hailstorm, the Doctor noting to Jo Grant in embarrassment after they rediscovered the ship that the setting for the HADS was slightly too high. (PROSE: The Suns of Caresh)
The Tenth Doctor tried to activate the HADS after Finn Dargo had hacked Emergency Program One and attempted to arrest the TARDIS' occupants. The move allowed the TARDIS to materialise in an unexpected place in Dargo's ship, giving Majenta Pryce a slight advantage over Dargo's Intersol team. (COMIC: The Crimson Hand)
While fiddling with various settings, the Eleventh Doctor re-enabled the HADS in time for it to activate and leave the Doctor and Clara Oswald in a sinking submarine near the North Pole in 1983. It reappeared a short while later, but at the South Pole instead of the North Pole. (TV: Cold War) This event would later be referenced by Clara when the Twelfth Doctor assured her that the TARDIS would "turn up somewhere" after the space shuttle it had materialised in fell into a canyon. (TV: Kill the Moon)
When the Daleks attempted to destroy the TARDIS, (TV: The Magician's Apprentice) the HADS activated making it look like it had been destroyed, but the Twelfth Doctor later rematerialised the TARDIS using his sonic sunglasses, summoning pieces of it from all around the room; this action was called the "Hostile Action Dispersal System". (TV: The Witch's Familiar)
Behind the scenes Edit
According to the Make Your Own Adventure with Doctor Who novel, Search for the Doctor, Drax's 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.
Likewise, the webcast Stealing Series 9 Scripts is not considered a valid source by this Wiki, but features another use of the HADS: this time, it is one of the many defence systems placed by Steven Moffat on his underground "secret writing lair" below 221B Baker Street.