This page contains significant spoilers from ArmA 3's 'The East Wind' and 'Apex Protocol' campaigns.
The Eastwind device is a CSAT weapon of mass destruction (WMD) that is believed to be capable of causing earthquakes.
Its exact date of origin is not known, though due to the brief tremors experienced during the events of the Prologue, it is possible that it was already halfway or close to completion more than year prior to The East Wind.
Its existence is a closely guarded secret, and the CSAT alliance will go to extreme lengths to prevent the device(s) from falling into enemy hands.
Their resolve to do so is demonstrated in the non-canon ending of The East Wind, when both the device and research facility falls into the hands of CTRG Group 14 Captain Scott Miller. The loss of Eastwind forces CSAT into launching a massive counterattack to retake the device. The resulting invasion leads to the destruction of an entire brigade of NATO forces, initiating the start of World War III.
In Apex Protocol, Capt. Miller reveals that he believes the device was used to cause the subsea earthquake which initiated a tsunami, wreaking havoc across the Tanoa Province of the Horizon Islands. Throughout the campaign, NATO intelligence suggests that the device produces a series of foreshocks as part of its arming sequence.
By the end of Apex Protocol, Miller secures Eastwind and arranges to have it airlifted to an undisclosed location. CSAT dispatches numerous black ops teams to retake the device but are unsuccessful. The fate of the device after its capture by CTRG remains unclear.
How the device exactly operates is not elaborated on in either of the campaigns. However, it is clear that the process has a significant startup delay since the device must be manually armed.
It also must be physically deployed on-site to have any effect and can't be used to trigger earthquakes remotely. But once armed, brief tectonic shockwaves are always a prelude to the actual earthquake.
One notable fact about Eastwind is that it is capable of causing earthquakes in regions of the Earth which do not naturally experience any seismic activity on a regular basis. Examples include both Altis and Tanoa, with the latter also triggering a tsunami in the aftermath of the initial quake.
Though it's never revealed publicly, Eastwind's military and geopolitical uses are shown to great effect throughout both campaigns in ArmA 3.
It forms the cornerstone of CSAT's so-called 'Apex Protocol' plans that seek to destabilise countries that have favourable relations to the West. By triggering Eastwind in a specific country, the natural disasters caused by it are used as a pretext for CSAT to provide humanitarian aid and relief (on top of chaos caused by Viper operatives covertly operating in the country). This then leads to the establishment of CSAT military bases in that country in exchange for continued support.
Because Eastwind's effects are practically impossible to trace back to their source, it can be used without fear of retaliation as opposed to the likes of nuclear WMDs.