From Bulbapedia, the community-driven Pokémon encyclopedia.
The DMA while not being operated
The Defense Mechanism of Alto Mare (Japanese: アルトマーレの古代の機械 Ancient Machine of Alto Mare), often abbreviated to DMA, is a machine that was featured in Pokémon Heroes: Latios & Latias. It is situated in the Alto Mare museum. It was built many centuries before the events of the movie to protect the city from danger.
In order to activate the DMA, Latios or Latias must be used as a power source for it to work. When it has power, the pillars starts to fall down into the floor. The Soul Dew is then placed in the DMA to start it.
Once activated, the control center will open up to control the DMA, allowing the user all its functions.
Oakley attempted to use the DMA to take control of Alto Mare and (in the dub only) ultimately the world. When Oakley used the machine, however, she was enraptured with all of her power and overused it. This temporary insanity is implied to be a side-effect of the DMA, that also caused both the Soul Dew and Latios to suffer, which is why it was sealed off to begin with and was considered not to be used.
While using the machine to attack Ash and Latias, however, the latter used Safeguard, causing the DMA to malfunction and trapping Annie and Oakley in the control center. During the credits of the movie, police could be seen arresting them. Although Oakley's plans were thwarted, the Soul Dew was ultimately destroyed.
Using sophisticated motion sensors in the DMA's control area, one can use hand signals to perform a variety of functions. It can create a lockdown by making metal barriers appear throughout the city and can even control water itself; bending it into the shape of the controller's hand motions. In addition, the DMA can also revive the Fossils of Aerodactyl and Kabutops within the museum.