From Halopedia, the Halo wiki
A piston is a Forerunner device. They 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.
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