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) The HADS had to be manually set, and the Doctor often forgot to do so. (PROSE: The Krotons)
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)
Behind the scenes
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.