Piston

A closed piston within the Sentinel Wall.

The Weapon: "Well... that's different. What now?"

John-117: "Down."

— The Weapon and John-117 after activating a piston which reveals a large hole in the ground.^[1]

A piston is a Forerunner device.^[1]^[2]

Overview

Pistons are installed within facilities like the Sentinel Wall as vertical "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 Aggressor 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 Sangheili, 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. Some pistons do not split in two, but the ones that reveal the winding tunnels just lift upwards, downwards and away.^[2]^[1]

Trivia

In Halo 2, an alternative for activating the pistons besides simply interacting with it is through meleeing or firing at the pistons. On Heroic or Legendary, pistons cannot be triggered, but must be shot or meleed. It takes four melees to fully activate the piston. The piston's holocontrols change shades from green-blue to dark blue as they take more damage.
Pistons are only present in the level Sacred Icon in Halo 2 and in the level Nexus in Halo Infinite.
In Halo 3: ODST, in the level Data Hive which is set in New Mombasa's Data Center, devices called "data stacks" are encountered. They function similarly to the pistons.