Piston

A piston is a device that the Forerunners, or perhaps the machines, left behind. They are installed within facilities like the Sentinel Wall as "doors" to get from level to level within the multi-tiered construct. The pistons have sensitive control-strips that open the piston when fired upon or switched. The piston then splits in two, and allows the Sentinels and Constructors to move to the lower or to the higher level they desire. The openings are fairly large, however, and therefore allow larger creatures, such as Elites, to move through. However, the shafts pistons guard are almost always vertical, so moving up through them is impossible without a thruster of some kind.

Trivia

 * On Heroic or Legendary, pistons cannot be triggered, but must be shot or meleed.
 * The piston's controls turns a darker shade of blue the more it's hit until it opens.