It appears in Halo: Combat Evolved and Halo 2, in the levels Pillar of Autumn and The Armory, respectively. Both times, it is used to bring online the MJOLNIR energy shields, although its primary purpose seems to be testing the shields in Halo 2.
First mentioned in the Halo: The Fall of Reach novel, during the MJOLNIR Mark V's first mission, this device charged the armor's shield before they were activated by John-117, like a battery. In Halo: Combat Evolved, the tester makes the same noise as a Plasma Pistol overcharge impact, possibly hinting that they were made using scavenged Covenant technology.
It is unknown why the shields need an initial charging. Cryostasis may be a logical explanation for Halo: Combat Evolved, but the MJOLNIR Mark VI in Halo 2 had no such explanation. It is possible that the fusion power cell needed to reach a certain internal temperature to begin it's reaction and the initial charging gives it the energy to do so, or the new suit still had to be started up fully.
- The UNSC Pillar of Autumn's tester has the same noise as an overcharged Plasma Pistol. This could be due to time constraints and a lack of a custom sound for the tester.
- It is possible to be standing out of the shield tester area in Halo: Combat Evolved and in Halo 2. However, you won't be able to move while the shield tester is active.