Tech-Priest Manipulus is a member of the Techpriest Auxilia, of the Adeptus Mechanicus. Usually a senior member of the Mechanicus' cult, the Manipulus is charged with different tasks, though they usually all revolve around combat and warfare. Most Manipulus are known as members of special operations teams. While they have access to very advanced gear and weaponry, they usually excel at close and short ranged combat, in contrast to other tech-priests, like the Dominus.
On the Fluff
The first Manipulus was first introduced as Balphamus Vaulk, the leader of Theta-7, a kill team dedicated to finish problems as part of different Imperial campaigns. Though it's well known that Balphamus Vaulk has his own plans and goals, his services are greatly appreciated considering the range of missions they can pull off, from assassination, to sabotage, to escort, to stealing materials and technologies... In particular his team has a focus on stealth, which he gets by hiring Sicarian Infiltrators or Ruststalkers (one is stealthy because you don't see it, other is stealthy because it can see you).
The Manipulus also houses what's known as a "galvanic cell" within their bodies, which is a mote of living electricity with which they use as the source of their powers. This galvanic cell means that they are the keepers of the Motive Force, 1/3rd of the Cult Mechancius's religious figures. In their duties, its said that they use their mechadendrites to drain things of their electricity, and used their stored Motive Force to ubercharge their buddies like a techno-cleric, overclocking their limbs and super-bumping their guns.
On the Tabletop
He's got almost an identical stat-line to the Dominus, just with a 3+ BS instead of 2+, the same 5+ invuln and self heal d3, as well as the ability to heal [Forge World] models d3 and Questor Mechanicus by 1. His unique ability, and the reason to take him, is his galvanic field: at the start of the movement phase he can either "Bolster Warriors" and increase the movement characteristic of [Forge World] models within 6" of him by 1, as well as boosting their advance and charge rolls by 1, or if he stands still he can "Bolster Weapons" and boost the range of ranged weapons with 24" range or higher by 6" and everything lower by 2" for [Forge World] models within 6". As for wargear, he's got mechadentrites for d6 free S user, AP0, D1 attacks, and an Omnissian staff (S+2 AP1 D2) and either the Magnarail Lance (18" Heavy 1 S7 AP3, Dd3 (straight D3 if he didn't move) or the Transonic Cannon, a S4 AP-1 D2 flamer.
In Kill Team, he's pretty much the same.