Originally developed to skitter across the battlefield and latch onto enemy vehicles and hardware to “hijack” them, the Jacker unit enables the tide of battle to swiftly change using a relatively low level of resources. Every vehicle gained is one lost to the enemy and the chaos stemming from hardware within their own ranks as it turns upon its forces is invaluable.
However, in the story their original design is twisted so the hardware they hijack is the human body. To keep the host alive and maintain homoeostasis for as long as possible (to prevent rigamortis from setting in and not waste the vast processor power needed to do it itself) the Jacker unit controls the host only from the neck down.
This would explain the look of terror and panic in most of the victim’s eyes, as they are unable to control their own body as they attack their comrades and can still feel their bodies pain. So if you’re having a bad day, think of these guys and it helps put things into perspective.